I'm on the same version of 16 as your signature and opaque background color works from number selector just fine.
Are you on Hotfix 3270?
Update:
Just installed 3270 and it has the bug, but I don't have the problem with 3014.
I can confirm that it does not work if you manually change the pen number in the text settings dialogue box.
If you pick the pen colour from the list or type the number in the info box then that works fine.
This is with the latest hotfix as well.
Barry.
One of the forum moderators. Versions 6.5 to 27 i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10 Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11