I would go with believing themelooks - it's a definite bug because it's listed as supposedly being fixed in the v8.2 beta changelog...
possibly they only tested in on the main 21 theme and not a child of it - the test to see if that's the case would be to use the main 21 theme on the beta and see if the issue is fixed... if so, then it's a case of reporting the bug / opening a ticket with WHMCS Support and letting them know the issue still exists with a child.... I would suspect the next beta/RC release will be around the corner, so soon er they know the better.
if you get your license directly from WHMCS (login required) - https://www.whmcs.com/support/
if you get your license from a reseller / host (no login required) - https://www.whmcs.com/reseller-support/
one other thing you could try, on both the beta and the main site, would be to copy the 21 version of supportticketsubmit-customfields.tpl to your child theme and see if that fixes the issue temporarily.