The whole 2 left footed centre backs being unbalanced is in marginal gains.
There are many right footers than left footers... so there are more right footed centre backs. As such, at some/several points during a right footed centre backs career, he will be asked to play as a left sided centre back (as his partner/s are also right footed)
Left footed centre backs (being rarer) will rarely be asked to play on the right, as it is probable that their partner/s throughout most of their career will be right footed.
So, in much the same way that right backs often look lost when played as conventional left backs (because they are not familiar with playing there), left footed centre backs playing as right sided centre backs is almost like learning a new position, as silly as that might sound, and can in theory lead to a decline/drop in their performance compared to when they play on the left
Hope that waffle made some sort of sense, lol
TLDR - if you are less familiar with playing in a certain position, then if you are played tehre, then your performance may drop (albeit slightly)