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It sounds to me like your key-reading routine doesn't have enough delay between writing to and reading from the key port. I'm not sure how much delay is safe enough for every calculator, but I know that Quigibo recently increased Axe's number of cycles between the port write and read for key input from 19 to 22 cycles because it turned out that 19 cycles isn't enough for some calculators running at 15MHz. Interestingly enough, Darl181 is the one who reported this problem for Axe as well.
This might be unnecessary, but are you writing $FF to the port when you are done? I know when I first switched to 15MHz I had to do that because my calculator would glitch otherwise. I might have done something wrong back then, but all of my programs write $FF after every read.