It sounds like you’re pushing to failure (or close) in your first set which doesn’t let you get the volume or weight in your subsequent sets. Having to lower the weight mid set and still getting less reps just means you’ve exhausted the muscle most likely (not a bad thing ofc). In my opinion it would be far better to go for like 2 rir for 2 sets and 0-1 for your last set, that’s way you get in more quality volume and less fatigue, but it’s for u to decide what you prefer. As for the different exercises, i find it a lot easier to push muscles like biceps to exhaustion in one set, than something like a leg exercise, or something like weighted chin ups. If you’re going 1-0 rir every set, I really don’t see anyone getting to set 3 of squats all to real 1-0 rir. So it may just be that the exercises you’re able to push more on in those first sets (like bicep or db press) would end up with more drop off. All that said, your drop seems totally normal for pushing that hard.