They happen when lines meet where the width of the feature isn’t an exact multiple of the extruder’s width and the printer has trouble filling in the void. Usually they’re fairly invisible, but when printing white on black, they stand out like a sore thumb.
I’m wondering if there’s a good simple way of avoid this problem in the slicer. The ultimate fix of course is to print a square sheet of white PLA under the white letter, but I’d rather not mess with my model because it’s quite complex already.
Use something other than FDM 3d printing - SLA (resin) printers have higher resolutions and will not have those issues. Or you could put a solid sheet of plastic on a mill and use subtractive machining to make your parts. There are a few other options.
All options have pros and cons, and your best answer might be just live with it (see other replies for ways to mitigate the issues). However don’t get locked into 3d printing just because it is the “in” thing.