Taxonomy is erased when adding item to new or existing node

As reported on #578354: Taxonomy is erased when adding item to new or existing node:

If a node is edited to add an attached file, regardless of whether it was previously created or not, the taxonomy is cleared and has to be reset. Something about that call to "add another item" wipes it out.

I'm experiencing this behaviour as well. (Issue duplicated because that one was pointedly ignored for bundling issues together.)