Thank you, was able to quickly debug and find the cause with your install. It's due to the check to see if the field is even supposed to output a formatted value to determine if it needs to use placeholder or just turn off ajaxing. This check fails against values of 0. I've fixed the check for next build and have already installed a fixed build on your install for you.
forge.joomlapolis.com/issues/6427
The workaround is as you've already discovered and it's to supply a placeholder value when configuring the ajax. This won't be necessary with the fix though.