Default [Q] Very odd SMS issue

Hey everyone, I'm not sure if one of you has experienced this but I have been having something very bizarre occurring with my text messaging. ONLY (so far as I know) between my mom and myself, text messages occasionally get truncated and concatenated with random old messages. For instance, I just sent her a message and what she received was the first half of my message and the second half of a message I sent over a year ago (from a different phone, no less)! She has had the same phone the whole time, but we've restored it several times and nothing's helped (the portion of the message that gets appended is not even on her phone anymore). It seems to only happen between my phone and hers. My texts to everyone else go through fine, her texts to me come through fine, and others' texts to her go through fine, but periodically, our exchanges are just garbled nonsense. Has anyone else experienced this or does anyone have any ideas on what could be going on? I'd greatly appreciate any input!

Oh, it might be noteworthy that this seems to only happen when I send messages from my computer, but I don't think this is the culprit because I've used multiple desktop SMS clients and it's happened on all of them (and still only with my mom).

Edit: I just looked at two of the "broken" texts that she forwarded back to me and both got spliced after 154 characters. This doesn't seem like a coincidence to me but being that it's under the 160 character SMS limit, it also seems like a rather odd number. Perhaps someone can make sense of it.

Edit 2: I sent my mom a lengthy message simply saying "test test test ..." and got another unusual result. The first one, which was greater than 160 characters, went through just fine, but the second, which was closer to 500 characters exhibited this peculiar issue...But this time, the message was as follows: "test test test...[after 154 characters] __PORTION OF RANDOM TEXT___ [back to] test test test ..." Does anyone have the slightest idea why this is happening? It's so odd and now I'm mostly just curious about what the **** is going on and why it's following this pattern.