A small team moderates the leaderboard for Minecraft Icarus speedrunning (https://www.speedrun.com/mc_icarus), we constantly run into countless problems with the leaderboard and visibility of runs. One of the main categories, the pre 1.16 leaderboard doesn't display any runs, even though we verified a good few of them. They all show on the user's individual accounts, but the leaderboard stays empty, and this is the common theme for the entire game, sometimes entire leaderboards of runs are missing and a lot of the time individual runs don't show up at all on other boards, like the current world record for 1.16 rsg is just missing. We tried to mess with all the settings of individual categories and variables, even swapped them around, and the entire board just appears to be bugged. Is there anything we can do/did wrong or is it on the side of the website/API

That is because the value "Easy" of the variable "Difficulty" is a default value.
Leaderboards are filtered by default values by default, so that's why you don't see any runs in the pre-1.16 - they all have normal or higher difficulty.
Remove the default property and it should be fixed.
(In general, the default property right now mostly works only with subcategories variables)

Dude, thank you so much