there are many people out there with funky dead lift form, and yet they lift heavy for years. my belief, based on mechanics, is that they are good at bracing. good bracing unloads the spine, with the consequence that a small deviation from neutral spinal alignment doesn’t cause harm because the spine is essentially unloaded. however, bad bracing even with the spine in perfect neutral alignment could cause serious injury due to so called Euler buckling from compressive and shear forces, in layman terms, a visit to snap city.
i wish i knew the best way to quickly learn how to brace well. would prolly have saved my back years ago. my best guess is to dead lift a lot and do it without a belt and really try to make the abs take the compressive load, the shear load will follow suit. alternatively, could be to try and take the shear load with the oblique abs, then the compressive loads will follow suit. my sensory feedback from the oblique abs is not very good. i go with the compressive force in the front abs.
i guess the take home message that i want to convey to you is, it’s good to try and keep the back flat but it is even better to brace.
actually, imo your back stiffens (good thing) after the initial rounding, maybe then you have found your bracing. if you experience discomfort you should definitely do something about the rounding but otherwise i’m not sure it’s necessary. what makes me worried though is that you don’t report that you feel it in your abs after.