If I edit a keyword in LrC, all photos with this keyword are synced (even photos where keyword sync hasn't been activated)

If you edit a keyword in Lightroom Classic (like adding synonyms or changing the keyword's export settings), all photos with said keyword are synced - even photos where keyword sync hasn't been activated.

Lightroom version: 15.4.1

Steps to reproduce:

1. Open a catalog that is synced to Lightroom cloud.
2. Activate keyword sync in LrC settings.
3. Activate keyword sync on a selection of photos in the catalog (but not all).
4. Choose a photo which doesn’t have keyword sync activated. Make sure that it has one or more LrC keywords.
5. Find the chosen photo on Lr Mobile / Cloud and add a few random keywords to it.
6. In LrC, edit one of the keywords of the chosen photo by adding a synonym to it.

Expected result:

Lightroom should sync any photos which have the edited keyword and keyword sync activated.

Actual result: 

Lightroom syncs ALL photos with the keyword, regardless of whether they have keyword sync activated or not. (This sync will have the effect of the random keywords added in step 5 to be synced back to LrC and added to the keyword hierarchy at the top level.)

Personally, this has caused me a massive headache as loads and loads of “flattened” keywords have been synced back to my Lightroom Classic catalogue from photos that I had not intended to sync keywords for.