Steps to reliably reproduce:
  1. Create slot, add weekly recurrence, lock to calendar
  2. (Wait for or manually trigger sync)
  3. Alt-move slot to another day, confirm "This and future time slots"
  4. (Wait for or manually trigger sync)
Result: Slot in original week will disappear from Akiflow but update correctly in Google Calendar (Note: these will never resync - it will stay in Google Calendar until deleted and never reappear in Akiflow). Slots in following weeks will stay on the original day, as though they had never been modified.
I have yet to do any comprehensive testing to see if this affects other items such as events/tasks, but changing other properties of a weekly recurring slot (e.g. renaming it) leads to similar behavior where the directly modified slot disappears and following slots are not updated.
I'm literally not sure how to make changes to recurring slots without creating a mess that can only be cleaned up by fully deleting all instances (in Akiflow and Google Calendar) and building it back from scratch. :(