Here's the gif. Date and Chart Score are both virtual tags, Peak Year and Billboard Singles are custom tags.
Date: $IsNull(<Peak Year>,$IsNull(<Year>,,<Year>),$If(<Grouping>="A Prairie Home Companion",<YYYY>,$First(<Peak Year>)))
Chart Score: $Split($Replace(<Billboard Singles>,*,),"Score: ",2)$If($Contains(<Billboard Singles>,Score)="F",$If($Contains(<Billboard Singles>,+)="T",[current],),)
As you can see in the clip, Date by itself sorts fine but as soon as I add Chart Score (holding Ctrl), date gets scrambled. Same thing happens if I use a custom sorting set.
Edit: I double checked that all the fields are correctly configured as date, number, etc. It looks like when I first add Chart Score it just sorts by Chart Score, ignoring date. If I click Chart Score again, though, it doesn't completely reverse it. It reverses all the 2 and 3 digit numbers, but the four digit numbers stay in the original order at the bottom of the list (with null values in between).