LookupValue not returning all values

I've come across an interesting behaviour with the LOOKUPVALUE DAX, where it is not returning all lookupvalues, even though it should. Has anyone seen this before? Effectively, I am using the LOOKUPVALUE to confirm if a value exists in both tables. (e.g. Does value 123, exist in Table A and in Table B). This works for tens of thousands of rows flawlessly.

However, the other day I stumbled on a couple values where they exist in both tables, but LOOKUPVALUE is not picking them up. This confirmed after I switched to a IF(TableA[Value], IN VALUES (TableB[Value]), 1, 0). The Lookupvalue formula gives me a total of 45,107 "matches", whereas the IF Formula gives me 45,466 "matches". A nearly 1% difference. Does anyone know what may be causing this behaviour? I have tried changing data type, etc. etc. and cannot figure out why lookupvalue is giving me blanks