Last week, around June 30 2018, this bug showed up from nowhere. It’s thus a problem in editable grids, related to currency numbers apparently.
After searching the web and finding nothing, I reached the conclusion that it was a Microsoft bug… not always easy to convince the customers though. However, since this bug is online and showed up right after the update to Dynamics 365 Version 1612 (188.8.131.520).
This is the link to the page describing the update in question, service update 14 for Dyamics 365 8.2.2.
And, on July 2nd, a new thread appeared on community.dynamics.com discussing the issue, only confirming it.
So as you can see, the only thing to do here is open a ticket to Microsoft support, and wait…
I hope they react quickly as for many customers, this is a show stopper for editable grids.