Might be either not enough activation or the joints not properly warmed up. I mean, literally, these are the only 2 reasons why performance would be better in later sets.
Try this strategy for fun…
Prior to your 1st workout set, do a heavier single.
For example in your example, your last preparation set is 85% x 1 (1 rep with your work sets weight).
After the 85% x 1, do 90-92.5% x 1 … then go back down to 85% for your first work set (rest normally after your set of 90-92,5%).
If suboptimal activation is the issue, this should fix it.
Gotta say, it’s really awesome for a coach of your caliber to be legitimately responding and following up to people’s questions on a forum like this. Lot of respect for the advice you give people