Setting the watched state in PHT causes PMS to reset "Recently Added" time data

I decided to manually spring-clean the watched states in my Movies library, but after doing so discovered that the Recently Added section is now all out of whack.

1. In PHT, I scrolled through Movies library and pressed "w" on the keyboard to alter the watched states of individual movies in the list.
2. In PMS web (and all clients - iOS and PHT), all the movies I marked as unwatched are now right at the top of Recently Added, saying they were "added 15 mins ago" etc. The ACTUAL recently added items are mixed in there, so they still work, but they're surrounded by 50-100 other items I just marked as unwatched...

Surely that's not how setting a watched state should work...

Both PHT and PMS are up to date.

Thanks!