Messages being read without reading them

I'm having an issue between my phone and my girlfriend's phone. Periodically (we text pretty frequently, and it happens probably every 15-30 minutes), when I send her a text (iMessage), it will show that she read it even though she didn't. She doesn't receive a notification for it, and when she goes into her messages, there's no badge on the app, and it shows that she already opened that message (even though she didn't). The same thing happens on my end with texts that she sends me; it says I read them, and I don't get the notification for them and they show up as already opened on my end.

This only happens on certain texts. If one of us sends more than one text in a row, only the first message shows up as read for the other person. It doesn't happen on every first text, just sometimes. Additionally, when it happens, it instantly shows up as read to the other person the moment they send it.

Our phones are completely separate from each other. We don't share an Apple ID or family, we don't have any accounts logged in in common on our phones, and we don't have any related names set in our contacts for each other (I've seen this pointed out as an issue before, but we don't have that). We don't have any additional devices that can access our iMessages. This only happens with each other; neither of us have had this issue with any of our other contacts. All of our notification settings are set as normal for the messages to show up on the home screen, etc, and they generally do for our conversation, it's just that every few minutes one shows up as read when it isn't.

She has an iPhone X. This was happening for me on my iPhone 8 formerly, and I just upgraded to an iPhone 12 (that I restored from the backup of my iPhone 8), and the problem is continuing. We both noticed the issue started happening around the time we upgraded to iOS 14.2, so we think the issue is tied to that. Does anyone have any ideas?