S20FE will not send long texts

[Kachow19]

My partner has been having this issue on their S20FE where they can't seem to send any texts over a certain character limit (I'm not sure what the limit is exactly, but it's not very high. It's definitely less than the 160 character limit of SMS). This started as an issue about a month ago with only a few contacts, but as since grown to not being able to send long texts to anybody. Texts underneath this limit send without issue, and photos also send without issue, the problem is strictly for text messages over a certain length. I've been trying for weeks to figure out what's going on here but I just have no clue. Here is what I've tried so far:

 

-Restarted the phone
-Cleared the messages cache
-Updated the phone/messaging apps
-Switched messaging apps (switched from Samsung Messages to Google Messages)
-Reset network settings

-Reset APN for mobile provider

-Swapped sim cards with mine, which confirmed that this was an issue with the phone since I didn't have the problem when I used her sim card. When I put my sim card in her phone though, she was still not able to send long texts.
-Enabled & Disabled RCS

-Fiddled with countless settings
-And finally, factory reset the phone

None of the above worked unfortunately. From what my partner told me, it all started at approximately the beginning of September and was working fine before hand. There was likely some update or something that caused the change, but I can't seem to find the specific dates of Samsung security updates anywhere, so it's hard to pinpoint it. At this point, I really have no clue what the issue is. I feel like I've tried everything and still have no clue what's wrong. If anyone here has had any similar issues, please do let me know or if you have anything to try, I'm all ears.

Thanks!