iPhone defaults my car stereo to bluetooth audio even when dock connector used

Hardware setup:

2016 Toyota Sienna, stock stereo (no CarPlay).

iPhone 8+ connected to stereo via USB (standard cable - USB <-> lightning connector on phone).

Prior behavior in iOs 10:

Phone is paired to car audio via bluetooth (both phone and "media player" options on Toyota stereo).

Phone calls via bluetooth work fine as designed.

Phone will play music through either bluetooth or dock connector - will default to last mode used. I generally listened via dock connector because music sounded better (better lows and volume control).

I almost always have the dock connector plugged in for this reason, as well as to charge the phone while running errands and the like.

When engaging Siri to find songs, play songs, etc., she would respond through the head unit via bluetooth like a phone call.

"Hey Siri, play Trolls soundtrack" (I have two young kids, I know...)

Siri would respond to her finding the album and give her introduction through Bluetooth. The iPhone would play the media through the dock connector. The van head unit would also stay in USB mode and there was complete hands free use of this feature - which was awesome for someone trying not to crash his manivan full of children.

After upgrade to iOs 11, the behavior has changed (and I verified there have been NO updates to the Toyota head unit through checking the software version - it's the same since I purchased the vehicle two years ago, and Toyota has not updated the software otherwise. I have also not changed any settings on the head unit either).

Same setup as before, phone pairs to head unit via bluetooth for both phone and media player.

Phone calls via bluetooth still work as desinged - no problems.

Now, when engaging Siri to play songs., etc., she will stay on the default output mode like before (if she was playing in dock connector mode when engaged, she returns iPhone to dock connector mode when done speaking to me via bluetooth, and vice-versa - if she was playing music through bluetooth mode when engaged, she returns to bluetooth mode).

However, when she's done responding to me and the head unit returns to play music, it does not return to previous input (USB/dock connector) - it now always defaults to Bluetooth to continue playing the media.

The head unit has not had any software updates, and this immediately started happening after the update to iOs 11.

So, to recap, here's the (unexpected) behavior after the update to iOs 11:

Listening to Coco soundtrack (kids, remember?) via USB on head unit.

Dad gets tired of happy bouncy music, and decrees:

"Hey Siri, play Straight Outta Compton soundtrack" (the kids have been dropped off - settle down).

Siri responds via bluetooth (it's actually labelled "Mobile Assistant" on Toyota head units - same behavior as before) and says something to the effect of "You are the coolest dad anywhere" and begins to play the street knowledge via the dock connector.

However, the iPhone somehow forces the head unit to switch to bluetooth mode (new/unwanted behavior), which is silent because the iPhone is outputting those beats via the dock connector.

Now I have to somehow open the Music app, or flick up the control panel, and manually switch the output to bluetooth, or I have to change the input manually on the head unit back to USB - when this did not happen prior to iOs 11.

It's odd, because it's actually the head unit that appears to not behave, but there is no logical reason to put this on Toyota as there have been NO changes (either settings or software). I noticed this behavior on the first day I updated to iOs 11 and finally have come here to beg your help to fix this issue.

Now, you're going to want to say:

1. "It's not Apple, it's Toyota."

I respond - nope, there have been no changes at all to the head unit software or settings.

2. "Just learn to like music through bluetooth, and all is fine. If the phone and head unit are both in bluetooth mode and it defaults to bluetooth mode after using Siri, just suck it up, Buttercup!"

I respond - sure, that's possible, but I lose two very important things I enjoy by using the bluetooth connection for music.

A. Bluetooth audio pales in comparison to a direct connection and the audio levels are noticeably lower as well.

B. When in USB mode, I have trained myself to pull the dock connector from the phone before turning the manivan off so the head unit goes into "Audio Off" mode. This prevents the audio from autoplaying when I return to the van ( which is nice when you've forgotten that the last song you were listening to was track 2...

3. "Stop connecting your phone via bluetooth for the Media Player function."

I have tried this, but once the phone reverts to bluetooth mode (after using Siri), the phone autoconnects to the car and I can not "unpair" this feature. If pair the phone to use handsfree for the phone, it is also going to pair via media player when it gets a request to use the media player - so I just allow it to connect from the start because there's no way to separate it. And remember, prior to iOs 11, this WAS NOT AN ISSUE!

So, anyone smarter than me able to troubleshoot this problem?

What did Apple do in the iOs 11 update that causes my Toyota head unit to think it needs to switch to bluetooth after using Siri!?