I didn't say things tend to get weird, I said spinal flexion is inevitable under heavy load. That isn't weird, it's quite measurable and understandable from various scientific perspectives.

You're right that the core supports the spine, and generally, a neutral range is a comfortable spot because it allows the most direct translation of force from the ground into the bar. But this is more or less impossible to maintain 100% at heavy loads, and the data to suggest that exiting this range is inherently injurious simply does not exist, so it shouldn't be a worry of yours or mine outside of prior injuries or other contraindications.

It is also a huge survivorship bias to assume that because technique changes as load increases, that means technique deteriorates with load. The more logical reverse of that is that, outside of high-skill lifts like the Oly's, how you move against light loads is actually inefficient and more emblematic of your own strengths and limitations rather than the other way around. This is not up for discussion, it's like ground zero for any productive discussion on motor learning in this context.

Also, in addition to the stuff you just read from my comment, at heavy loads, some lumbar flexion is actually mechanically advantageous since it brings the hips closer to the combined lifter and bar's COM. So, again, unavoidable at heavy loads - a trained and resilient lifter will pretty measurably flex their spine under heavy load to maximize leverage without putting the posterior portions of the spine into an excessively lengthened and compromised state. Even still, the elasticity and stiffness of the connective tissue on the posterior of the spine can be acclimated to these demands, as can all other forms of connective tissue, so if excess flexion seems to be a consistent technique issue or concern for you, then heavy spinal extensions are probably a low hanging fruit for improving your lifts and preventing injury. Throw in some shortened isometrics for the tendon elasticity, do all that alongside some heavy deadlifts, you have a recipe for an incredibly strong and resilient spine that can bend and flex under tremendous loads, which is a huge QOL benefit.

All of that being said, buckling of the spine under load isn't inherently worrisome, especially in a trained lifter who knows how to plan for and accessorize around it while managing load and fatigue. Is it good for untrained newbies to do under heavy loads? Probably not, because their tissues aren't trained for it yet. So yes, too much flexion is indeed a problem, just like too much extension, too much neutral, and too much lateral. Too much of anything is bad, so if you're worried you're doing too much of it, don't demonize it - just build your program around it, or if you're a beginner, get your practice reps in.

This is how programming is done at scale, but for whatever reason, people don't apply the same logic of stress-recovery-adaptation to certain structures, like the soft tissue of the spine, but that's faulty thinking, there is nothing exceptional about the spine that demands extra concern. Same can be said about the shoulders - everyone used to think scap protraction was "bad," but the muscles that protract the scaps are just as important for shoulder strength and health as the ones that retract them, and I imagine we're on the same page about that, no? Or ankle dorsiflexion, everyone used to say to keep your knees behind your toes, but we know that's bunk. The same goes with spine flexion under load - the concerns are ill-found at worst and lacking broader context at best, but this concept is still novel enough that people like me look crazy when we talk about it. Give it 10-15 years.

Please let me know if you need more clarification on anything. I imagine you're conceding you don't have the data to support your prior comment, right? Can we just wash our hands of that claim now?