One other thing to check...what version of Tableau are you using? There was another Tableau forums thread (I don't have the link, sorry) where a FIXED calc wasn't acting like a fixed calc in some contexts, and Tableau confirmed it as a bug.
Ahh I thought this might have been the case as I was using v9.2.0 - but I've just upgraded to 9.3 and this issue is still on-going!
Thanks for getting back to me so quickly though!
Did you resolve this? I am having the same issue with my LOD calculation.