Alert for the issue:
https://community.quicken.com/discussion/7927815/new-1-17-23-categories-disappearing-in-split-transactions/p1?new=1
Current workaround for this issue is to revert to a 44.xx version. See this post for details on reverting to a prior Quicken program version.
https://community.quicken.com/discussion/7928128/reverting-to-older-version#latest
Note you can use your current Quicken data file with the older program version.