I am just a user, but I would like to comment on some issues that maybe help you (while you expect an official response from Memrise Support).
For the past 60 days, I have not read any official topic stating that chatbots or grammarbots would be modified. But MemriseMatty reported that in the app was created a Grammar Sessions on Android (click here), as you already noticed.
I noticed that the initial modules of the official courses of most languages were changed about a month ago (for British English speakers). French, German, Japanese and Italian are some of them.
The official courses usually have 7 modules, and modules 1-2 have been changed (in the case of Japanese, module 0 added as well). But the web version of the new courses had some problems. I reported some of the problems here.
I do not have PRO so I can not test, but maybe the problems with chatbots and grammabots are related to the new courses. Or maybe Memrise is thinking of modifying the system of the new courses?
You could test with modules that have not been exchanged and have (or had?) chatbots / grammabots, such as module 3 of those courses. Or the old version of these courses. Old versions can no longer be seen on the course page, only new versions. If you do not have them, you need to ask people who added the old versions before the update. Or get an old link in the forum.