Hi Mark,
Checking back on this, I tried to reproduce the bug described here with a variety of topics selected via search through the bulk actions menu, but was I unable to get this behavior.
Selecting searched topics with the “Select All” button, and then using the bulk actions menu to change the categories or tags of the topics successfully changed the categories or tags of all topics each time I tested this.
I also did not experience this with any of the topics I tested.
Based on this testing, you should no longer be experiencing this bug anymore.