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.
Im only familiar with Cura. Cura has wall settings I think you are looking for to fix your problem. I had similar issues with a print. Used settings like wall transition length, wall distribution count, wall transitioning threshold angle, wall transitioning filter margin, and minimum wall line width. I was able to fill in tight corners, just took a little while dialing in the settings.