Conditional Format Based on a Defined Range

I'm unsure as to how those functions work (MATCH and F6 specifically), but it seems very random as to what values it affects

I changed the formula to
=NOT(ISNUMBER(MATCH(F6,PrimaryVendor,0))) to find exact matches

The formula now checks for an exact 'Match' based on the input in the active cell (F6 in your first case) with your 'PrimaryVendor' list . Note the relative reference to F6, this will change to F7, F8 and so on for the next rows. If it finds a match it will return a number which will be identified as a TRUE by the ISNUMBER function, the NOT at the beginning will convert this TRUE to a FALSE and the condition will not be met, thereby not changing the format of the cell.

However, if it does not find a match, the ISNUMBER function will return a FALSE which will then be converted to a TRUE and the format will change.

It does not work on a random basis as the format will change only for values that are not found in your PrimaryVendor list.

Hope this helps! See attached.