I've seen 2 things happen that may/may not be related.

Probably related:

If you have 2 Google accounts that you use simultaneously and I logged out of one to access another, I've had where I accidentally entered calendar info on to an open window that previously had the correct LOGGED-IN Google account. When I finish the entry and then save it, it doesn't save and there is no info there as I logged out previously. Is this a possibility?

Probably not related but wacky:

I started seeing 18-24 months ago where client info doesn't show up when I search for it. We have to sometimes search in creative ways to find it now. Ex: In the past searching by the last name brought up all past visits. Now the same search only brings up 50% of the entries. However, if I search by a different phrase (ex: that customer's street name) I may see ALL of the entries come up. I don't know what that's about. Any help would be appreciated.