I've checked the hardlight panties, panties + stockings, and boxers on a mix of Penny (not crew, no additional mods), Saendra, Anno, and some Vanae (where I first noticed the issue), no scenes using the strap-on are showing for me. I can only presume this is a rare bug caused by some strange interaction, as I did some very light testing on a fresh save and didn't see the same problem. Save file attached.