Started happening today for me as well - or at least I started noticing it today and and after reading @hickbilly’s topic on tooltip problems (see Relic Explorer help) with the relic explorer (which I can confirm and also seems to affect talent tooltips) I got curious.
Apparently the location of wowdb’s tooltip css file has changed and the returned 404 error is delivered without https. As far as I can tell, the current location for the wowdb css file is https://static-azeroth.cursecdn.com/Content/1-0-99-0/Skins/Azeroth/css/tooltip.css. Might be a good idea to ask the people over at wowdb/curse about that change though.