The caveat was revealed on the Technical Specifications page for the new iPhone 13 Pro. Additionally, the page reveals that ProRes recording won't be available at launch; rather, it is "coming later this fall".
ProRes video recording up to 4K at 30 fps (1080p at 30 fps for 128GB storage)*
* Coming later this fall.
It's unclear why Apple is limiting 4K ProRes to models with at least 256GB of storage or whether that could change. For now, if you see yourself using 4K ProRes, make sure to purchase either a 256GB, 512GB, or 1TB model.
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