There are two editor window fonts: SBCS/DBCS Source Windows and Unicode Source Windows
Tools > Options > Appearance > Fonts > ...
You are probably changing one but your code is identified as the other (a lot of source files have sneakily became Unicode in recent history).
There are also a few others, File Manager Windows, Hex Source Windows, and Diff Windows, just in case if you want to make sure SlickEdit uses your favorite font everywhere.
Funny story. I added the Diff* Source Windows font options because, back when monitors were smaller and my eyesight was better, I liked to use a smaller font for Diff so that I could see more columns side-by-side. Now that I have a 4k monitor, I've flip-flopped that and I use an extra-large font for Diff so that I can see all the details very clearly when I am reviewing changes.