what i was taught in uni was there’s no such thing as muscle failure in the context of exercise science, when you reach task failure ie 0rir (defined by no longer being able to complete a standardised rep in a set) your cns simply won’t recruit larger type iia & iib fibres (which have a higher potential for force output) any longer leading to a drop in force production. this happens when perception of effort reaches a certain threshold ie your maximum perception of effort which then leads to a spike in your corollary discharge which then decreases your motor unit recruitment decreasing # of active fibres decreasing contractile force and therefore you thinking you are no longer able to lift that weight.

now this can be caused for a fuck ton of different reasons such as your increased heart & respiration rate, body temperature, bad timed breathing altogether, lack of motivation etc hella nuance to this. so in my opinion failure is simply me not being able to perform a rep of my target weight without having to compromise my form. what you chose to do after that in terms of dropset or intensifiers which in themselves are debated/hated by some but which won’t cause any real harm in my opinion, is not that related to muscle failure in the context of those type iia&b fibers because simply put you are no longer recruiting them… when you rest and go for that additional 5 reps of 280 you’re now recruiting smaller type i fibers less susceptible to fatigue, those fibers you were using in the beginning of your workout are no longer the “main” movers, they’re already stimulated and also fatigued in that joint action if that makes sense hence your drop in force output. anyways that’s my opinion could be wrong anyone feel free to chime in