Welcome to T-Mobile and our Support Community! Sorry I didn't chime in earlier, but I did figure out what's likely going on with your messaging. We're aware of text messages being delivered out of order and our engineers are working on a fix. I don't have an idea of when a fix will be in place, but if you have to call in to check the status of the issue, you can refer to master trouble ticket 25795749.
This has been happening to me for years.
I did notice that it was from my Galaxy S9 to iphone users on Verizon.
Is it my android to iphone or tmobile to verizon? The texts aren't even exceeding 60 characters.
My seemingly regular texts are coming in (1/2) (2/2) and out of order.