I can’t give you instructions to reproduce it, but I thought I’d report it so you are aware.
I have a fairly complex field group that I would tweak, test, tweak, test etc.
Occasionally when I went to test one or more of the Select fields would lose their default. When I’d go back to check that field, it’s “Default Value” would be set to “0” (which of course was not one of my choices).
Could you please share some screenshots of the issue? Also, could you please share the JSON or XML export file of your field group so I can test it out on my installation? Please let me know how did you find the issue too (the steps that mostly will reproduce the issue).
It’s possible that there’s a conflict on your site. In this case, could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Sixteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.