When I try to add a new row in the subform, the line(s) above it blank out. If I complete the entry and click on the row above or tab out of this line to go to the line below, the line(s) show data again. Adding more rows does not appear to blank out data, just the first new add.
I have confirmed that this does not happen for non-Dataverse tables. I tried both local tables and a linked (to SQL) table and this was not the same behavior.
I suggest you report to him that I can duplicate this and that I think it is a bug which I will report to Microsoft. I will let you know if I hear anything back from them. In the meantime, it appears to be like you said, a temporary display problem so the user should be warned and advised to ignore as the line(s) should magically reappear as the row is completed and the user moves to a new or existing row.