I've been having this problem for a few days too, seemingly since the last google photos app update. A simple workaround that I've found is to go into the "device folders" tab and then you can edit the pictures as normal... It's a bit inconvenient, but at least we can still edit pictures until they fix this bug.
I have the same using One Plus 2 and I think this happened after the last OS update. Really annoying and clearly a bug. It can be bypassed with the solution proposed already, to go to the Albums and edit the photos from there. Thanks for the recommendation! It has to be sorted in the OS though.