There definitely have been cases where fares/availability have been updated, but where Google Flights is still showing old fare information (likely due to caching). This is particularly true with mistake fares where the airlines pull them rapidly, but there is a lag in getting it out to GDS systems. It also seems to be true with short-lived fare sales where you can still see dates with fare sale pricing showing on Google Flights, but when you go to actual booking sites, the fares are clearly no longer there. Although if you are regularly seeing changing at same time everyday, that's not what is likely going on here.