I have come across this issue numerous times, where more complicated queries that also involve the dungeon name, such as cos or sbg or hov only show as few as 10% of the valid results. Doing the same query while putting the dungeon name in the Blizzard UI search bar can result in 10x as many results.

There seems to be a bug where the addon filters out completely valid options when the dungeon names are involved. See an example below, the two screenshots were taken just a few seconds apart.

Well written issue, but please read the FAQ:

https://github.com/0xbs/premade-groups-filter/wiki/FAQ#why-do-i-only-see-a-few-groups-when-filtering-for-multiple-dungeons