Using itemID-s instead of names is a good idea. Saw that could be done but didn't want to overwhelm.
While still debugging, you might want to print itemId.
Don't know much of regular expressions but does string:match("item
%d+):") recognize that instead of [item]x2 you want to record [item]? I think it does but you might need to test. Does it detect that if you roll for item but loose, and doesn't record?
Function Log_Loot isn't local.
You are not using variables after player, so they can be omitted.
I think players from another realm have "-realmname". You can test it by putting "else print(player)" after line 9, and running a random heroic/mythic without premade group.