could be many reasons, software is complex. A fix they put in may not work as expected, may create other bugs, beta testers may have seen a last minute issue, it may have taken more time than expected to code various fixes, etc.
since it appears to have popped up at the last minute, I am leaning more to a bug fix not working as expected or beta testers seeing a new unexpected side effect.
I worked as a beta tester before and you would be amazed how many issues are only spotted at the last minute, even with the best of intentions.
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