iPhone goes to speaker during call when notification banner appears

When I'm on a call on my iPhone Xs Max (iOS v13.3.1) and a notification banner is displayed, the call switches to speaker. Whenever this happens, I look at my phone and see that a notification banner is displayed. So far, it's always been when a Facebook Messenger text has arrived, but I don't know if this happens with other banners, such as Apple Messages or a new banner.

Today, along with going to speaker, at the same time it also tried to initiate a FaceTime call to the person I was talking to.

I know it's not a problem with the proximity sensor, and I have VoiceOver turned off.

As far as I know, this just started happening in March 2020. It could be that the bug was there all along, but I just now started receiving Messenger messages while talking on the phone. I doubt it though.

I haven't seen any other reports of this happening, but it's unlikely that I'm the only one. Has anyone else seen this?