I thought I had some strange issue with my PC but then I saw a colleague have exactly same issue on a teams screen share when I was helping him out with data actions.
When typing in schema field names the UI randomly starts removing them as you type.
For example I type: ExpiryDate and the UI randomly removes the i and final e so I get ExpryDat
This happens with almost all entry and if miss it, you get a mistyped field which you can't directly change once published. I have to copy the whole action, give a new version name and fix the UI generated typos.
There seems to be no pattern to this, it's quite random. The same schema field name typed again may be fine.
I know it happens to me and colleague so assume others have had this happen to them, and thought they were going crazy!