I was wondering, if I had the mod "Returnable Masks, Patterns & Materials" active. Would this prevent a recording being valid? Or would I just need to show through the Beardlib tab that this is the only mod I have?

Mod: https://modworkshop.net/mydownloads.php?action=view_down&did=15940

Yeah, you shouldn't be using any mod whatsoever, otherwise your run becomes invalid. When you want to speedrun the game, remove them from your game files, disabling them in-game isn't enough. This rule is harsh but it has too in order to avoid creating double standards and more differences between PC and console versions.

Surely most runners including myself have forgotten at least once to remove their mods during a speedrun session. Just hope that doesn't happen to you when you are on a PB pace...