When you learn a hull mod of a new type, the refit screen hides that type; your new hull mod is invisible until you enable it in the filter list at the bottom. It would be nice to have them visible by default.
I totally didn't forget that those filters exist and spend an hour trying to figure out why the game was eating my hull mod specs
Note: This is only relevant to modded games where your installed mods add hull mods with different types, since in vanilla Starsector there is only one type of hull mod.