When I swipe “don’t” in Gboard, at least 30% of the time it decides I mean “didn’t”. I just tested it, and the accuracy was shockingly bad. I’d understand if the strokes were very similar, but “didn’t” has a whole extra stroke in it compared to “don’t”. WTF, Google?
I tried it on mine, but couldn’t get it to happen. It’s possible that Gboard might be trying to make a grammatical correction. Not to imply that you don’t know how to write, but something that needs to be asked from a troubleshooting perspective: Are you sure you’re using the correct word for the sentence?
Also, and this might be an annoying step to take but, does it continue after clearing the Gboard app settings completely? I think Gboard will also make “habitual” corrections based on your usage, so it could also be possible that it thinks you frequently delete “don’t” and replace it with “didn’t”, and is trying to preemptively correct it.