Maybe there is a RAM requirment and 128 GB Pro has less ram (4/6?) than higher storage device (6/8?).
I don't think so but IMO it's the only hardware related possible reason. If ram is same as 256GB, it is an intended software limitation.
I bet it is just a basic upselling tactic exactly why they still sell the watch S3 (to make you clearer the benefits of SE/7 watch model to sell more expansive model du to huge physicial differences you can see in 2sec)
Anyway with this ProRes thing Lighting gets ever more ridiculous escpecially when they are explaining its benefits few minutes before for iPad Mini. It's embarassiing.
It's more a flex phone than a "pro" phone at this stage...
+ cinematic mode capped at 1080p at 30 FPS, that is so bad 😂