iPhone 15 Plus iOS 18.11 Footage blown out on Premiere

That looks to me like it is improperly mapped to Rec.709.

So ... back to my previous post ... you say you have DCM, auto detect log, and auto tonemapping on, sequence CM Rec.709. And used a standard Rec.709 export preset.

But the images don't look like I would expect. "Blown out Rec.709" doesn't look like it's been tonemapped from HLG to Rec.709. Which is puzzling.

And the QuickTime ... is that the original clip shown in Qt player?

You simply need to set the color management correctly.  But you have to set several things.

Go to the Lumetri panel Settings tab ... the tab NAMED Settings. That's where all Premiere CM options now live.

Make sure Display Color Management, Extended Dynamic Range (this last only if on a Mac), auto log, and auto tonemapping are all on. Set the Sequence to Rec.709. And only start with export presets without HLG or PQ in the preset name.

You should be fine.

Viewing Gamma... thanks to Apple's stupid improper Rec.709 display transform, is a personal choice for Mac users. Pick your poison thing.

Most users concerned with how your vid looks in general should use Gamma 2.2/web. NOT because of going to the web though!

Use gamma 2.4 ONLY if working in a nearly dark room while grading. That is what colorists are required to use.

Everyone working in a normally lit room, by the same standards, is required to use a display transform while grading of essentially gamma 2.2.