Fixed it locally, what appears to be a case where the `SELECT` element is not visible/displayed when the plug-in searches for it, which results in an exception and causes the Spinner/Loading to not be released.
Fix: https://github.com/jenkinsci/active-choices-plugin/pull/425
I've attached uno-choice-JENKINS-74963.hpi, which is the plug-in built with the branch of this pull request. When I have time over next days I will see if I can write a unit &/| e2e test, but if someone else could either check the code or try this version and confirm it fixes this bug, then it would be a lot easier and faster to merge and release a new version with the fix.
Cheers
BTW, Jenkins 2.488 and Active Choices Plug-in 2.8.3, the result is ok~