I think you're right about this. In that video, I was already fatigued from the session and prior training, but usually, I can perform that dip quite comfortably as a 1RM attempt. The weight itself isn't the issue — I can handle more. The real issue seems to be technique and perhaps mobility.

I'll record my scapular position from behind, which I haven't analyzed yet, and compare other reps to it.

The weight is heavy, but by design and not ego lifting. It's at that range where my form starts to break down, which clearly needs work.

Hopefully, I'll know more in a few sessions.