Losing/dropping the .IDX file?

I backup the data every time I close out, but when I go to open the file again, I get a MS OneDrive notification about my XXX.IDX file being in the recycle bin.

I look there and it seems every time I open Reckon it send the .idx to be recycled.

If I open a different file, I'm not able to open my main file again, I have to restore it from the last back up. I'm guessing it's because of the missing .IDX file.

How do I stop this from happening?