I assume by “Most Common Surnames block” you really mean the list of most common surnames included within the Statistics block (as shown in attached image)?
That is indeed an issue from way back in PGV days.One day perhaps it should be re-written, or maybe even removed. The “Top surnames” block, which is a ‘real’ independent block works far better.
But for a fix to your list, have a look in Administration > Family trees > your family tree > Layout tab. I believe you will find your upper-case names listed in the field for “Names to add to common surnames” in upper case. Either delete them or change them to proper case.