I tried to configure the HS121 2E and there seems to be an issue with sizing business vs economy seats.
If I pick 20 standard C (Business) seats, I can fit 84 standard economy seats.
Given standard seats have more pitch than high density, the problem seems to come in if you pick 20 High density C seats, you can still only do 84 standard economy seats.
My experience with other planes is that in this equation, choosing 20 high density C over 20 standard C should let you maybe have 90 to 96 Y seats (not sure the HS121 is 5 or 6 per roe)...but you see the point.
Can you check if this is a bug or some reason there is no impact to Y seating based on type of C seat chosen?