Save for Web Ruining GIF Quality

Hello,

I've been having this issue ever since I updated my Mac to Catalina, which I didn't know at the time was a bad idea. It's been a couple months and I haven't been able to figure out the issue, and I'm hoping someone could assist me. 

Whenever I create a GIF, and go to Save for Web, it'll end up ruining the quality. Here is a (zoomed in) example of what my GIF should look like vs. how it ends up looking like when I go to save it:

My settings haven't changed:

Sometimes, I'll be able to create a GIF with absolutely no issue and the next time I'll go try and save one, it'll end up looking like that. 

Any help would be greatly appreciate.

Thank you!