Confirmed that I see this same issue on Jenkins 2.277.1 release candidate in one case and cannot see it in another case. I don't yet understand the difference in the two cases.
I see it consistently when I add the additional behavior "Clean before checkout" to the git plugin configuration in the Pipeline Syntax snippet generator. The "Clean before checkout" extension has help and includes a help icon, but when I click the help icon, the text does not appear.
The same happens for "Clean after checkout".
When I add the additional behavior "Advanced clone behaviours", the first help icon in that extension expands and contracts as expected when I click the '?' iccon.
I also confirmed that Jenkins 2.263.4 does not have the problem. This appears to be a regression between 2.263.4 and 2.277.1-rc