I just noticed that some sub-categories are not showing up in the default achievement frame when the addon is enabled. Notably, the only sub-categories showing up under the Feats of Strength category are "Mounts" and "PvP", when there should be five additional sub-categories.

When I use normal means to open the default achievements tab, I see all the sub-categories the same with or without KAF enabled. In your screenshot the default achievements tab is not selected in the bottom row so somehow you got there which might cause this issue.

What I had opened is the default Blizzard achievement window, which opens when I hit my achievement window keybind. If I click on the "Achievements" tab at the bottom, it switches to the addon's version.

Ok, I can reproduce the behavior you're mentioning. Blizz might've made some changes it looks like cause the filter button for me is also acting up. Probably nothing I can fix anytime soon but rebinding the achievement button to the KAF achievements tab should resolve your issue.