Problem with ScreenTips appearing dark for SharePoint Quick Links

The ScreenTips that pop up when I hover over a Quick Link in a SharePoint Online Communication site have been behaving fine for the past number of months. In the last week or so I added a new section to the top of some pages and applied the 'Strong' background shading to just that section on those pages. I'm not sure if the issue is related but I now have a problem whereby the ScreenTips appear with a dark fill and dark text. They are pretty much illegible and are visually unpleasant. 

I've opened the site in Edge and Chrome and on two different computers and the problem is there in all those scenarios. I tried creating a new site to check if the problem was specific to the original site, but the same behaviour was present on the new site too.

I tried using 'Change the look' to try and reset the display settings for the site but it didn't solve the problem.

Any help in fixing this would be greatly appreciated.