you for reaching out on the community and telling us about your issue. I do apologize for the issue you're having. I've taken a look on my side to see if I was able to replicate the issue and was able to. This has been submitted for a bug report under ticket CTP-1540. In the mean time I would recommend selecting the transactions then selecting edit transactions. From here you should be able to change the category without wiping out the tag field although a bit slower. I do apologize for any inconvenience this has caused.
It looks like you're new here. If you want to get involved, click one of these buttons!