I was having the same issue, and Joskule's answer helped me!
I don't think this is caused by a bug, but a new feature that defaults to this behavior. Since Archicad 25 you have more control over merging values (e.g. you can merge IDs into a single cell, and they will all be listed in the merged cell, separated by commas). This is the only material I found about it:
So, if you list your values as usual, but as an extra step, click this icon next to the header of the field, and change it to only merge identical items, it should work as it used to (at least it did for me).