There is a few bugs with the strapon implementation. I suspect the trigger or event used to replace dicks with strapon is not universal.
Here is an hopefully extenssive list of the bugs i found.
If you undress any girl coded as dominant they get a dick. Most of the time said dick does transform into a strapon when sex is engage, but it would be nice to get a strapon from the get go.
BDSM 8-9-10 decoration, the demon spawn and stay as futa.
All Robot decoration stay as futa too (although, i guess all you can do with them is removing the balls )
Every threesome, foursome, etc ... the "main" girl get a strapon, all additional girls stay futa.
City:
Construction workers stay futa
Every car or truck driver stay futa
The Nurse stay futa
-1:
All the slime fucking valkyries are futa.
2:
All currently recording npc are futa.
4:
Lucy
7:
Every girls, no exception, stay futa.
8:
Blue Wolf is one of two case when engaging sex make both the duck and strapon appear at the same time.
The 3 extra curvy cow girl at the end of the stage. Like the Blue wolf, strapon and dick are use at the same time (except for the additional girl of the threesome, who stay futa)
9:
all Green girl are futa outside of sex
11:
All the girls in sucking torture machine.
The sucking torture machine also transform the player's strapon into a dick
16:
Futa giantess stay futa
Hope it is not too much work to fix this.