Recently my main tooltip's background became transparent.
I don't see any option to even change that and it makes things really hard to read.

Is this a bug?
If not how can I undo that?

It's caused by another addon, as far as I know. ElvUI is known to cause this problem, but there may be others. Please try disabling everything but Rarity to make sure it can be reproduced as a first measure.

I'm sorry, but I've never used ElvUI and this is definitely something you should report to the authors of that addon. If there is a fix or workaround I'm sure they would be the first to know.

After disabling ElvUI, the background is back to normal. But re-enabling ElvUI it's transparent again.
Is there any known setting in ElvUI that might causes this problem? I can't remember recent changes in the settings.

I'd like to keep my ElvUI, so I'm looking forward to hear from you, if any possible fixes exist.

Same problem here. Hopefully ElvUI can fix it!

It's very odd, I thought the ElvUI Ace3 skin was the culprit, but I can't reliably reproduce the problem after a reloadUI. Sometimes it's transparent, sometimes it isn't.

I asked the ElvUI devs about it here: https://www.tukui.org/forum/viewtopic.php?f=5&t=7472