did you try to add 1 more text replacement on one device and see if that new one is showing up on the other device?
and, then doing the same thing on the other device as well?
question: are you using apple's built in Keyboard or a 3rd party keyboard? maybe doesn't matter but when i had this issue before i remember i did a complete delete of all 3rd party keyboards and then brought them back and it somehow began to sync.