First few frames clobberedJust a heads-up on a reproducible "hiccup" - here ProRES recording - but I suspect it is a general hardware behaviour to varying degrees based on options selected - and not necessarily the codec used. The following data should not be regarded as more, than a general tendency varying with the actual exposure environment. The examples were "construed" to make certain, that the effect was seen.
I've recently mentioned, that "Cinematic mode" has a "nasty" side effect in some situations (compared to "standard mode" stabilization). It is easily reproduced.
In the following, assume 25fps and 1/25s, ProRES 422 (similar results with ProRES 422HQ) fully manual settings (no autofocus either), white balance locked from start. Internal recording in local folder (not Photo library). Choose a high ISO value (in my case, highlights would be blown at virtually any ISO).
Aim the camera at a bright window (purpose is to really blow the highlights). A few seconds recording will suffice nicely. Frame numbers are manually counted, frame by frame, in the sample imported into Davinci Resolve Studio.
60Hz flicker setting (Cinematic mode)The two frames will show near perfect exposure (in effect, camera is in automatic mode). The following two frames are exposed slightly "warmer". Following frame (5) will be nearing "blowout", the following frames will quickly adjust to "fully blown", and continue in a stable fashion from around frame 17.
Key frame rate data:Billedratemodus : Variabel
Billedfrekvens : 24,922 FPS
Minimum billedfrekvens : 20,007 FPS
Maksimum billedfrekvens : 25,021 FPS
50Hz flicker setting (Cinematic mode)First four frames will show near perfect exposure. The following frame "varmer", then same development toward a stable "blowout" around frame 25. Difference to 60Hz flicker compensation may not be significant.
Key frame rate data:Billedratemodus : Variabel
Billedfrekvens : 24,927 FPS
Minimum billedfrekvens : 20,007 FPS
Maksimum billedfrekvens : 25,021 FPS
50Hz flicker setting (Standard mode)Stable from first frame (fully "blown").
Key frame rate data:Billedratemodus : Variabel
Billedfrekvens : 25,000 FPS
Minimum billedfrekvens : 24,990 FPS
Maksimum billedfrekvens : 25,021 FPS
The "Standard mode" stabilization is very near constant frame rate. The "Cinematic mode" stabilization introduces some wild speed variations, as well as hugely varying content (in fully manual mode, specified in the start of this post).
Note, that in all three samples, the metadata were identical:
com.blackmagic-design.shutterMode : Speed
com.blackmagic-design.shutterSpeed : 1/25
com.blackmagic-design.camera.iso : 568
com.blackmagic-design.camera.whiteBalanc : 7220
I have other examples of "Cinematic mode", in fully automatic, 25fps, 1/25s (the After Snow, After dark sample in an earlier post), where the original ProRES file exhibited these data:
Billedratemodus : Variabel
Billedfrekvens : 25,000 FPS
Minimum billedfrekvens : 24,979 FPS
Maksimum billedfrekvens : 25,031 FPS
Since automatic mode was selected from start, no problems arose in the first frames.
I assume, that the "processing pipeline" was less affected in this case, hence the system as a whole was less stressed from the beginning.
I have not (yet) established, if it is a case of only the first frame(s) being affected. Neither have I tested, if the last single or few frames in a recording can vary also, and if so, degree also depending on settings.
Maybe, just maybe, the VBR status of files is depending on settings, and just especially pronounced by the selection of "Cinematic mode" stabilisation. Other settings may also have some significance here, but the "blowout" samples (easily reproduced by you), indicates, that we may also see initial "flutter" in the frame rate in the first few frames of a recording ("flutter" affected by some initial "heavy lifting" from an "initial" default (semi-)automatic exposure state).
I'm looking for a tool, that makes it possible to extract imbedded timestamps of a ProRES recording, but until then: "beware of the gremlins"

This behaviour is probably caused by the Apple hardware not always able to get up to speed in all settings, initially "dropping" the correct timing for the first one or two frames (ideally) or worse in some situations. Both Cinema P3 and Blackmagic Camera exhibit similar behaviour.
If you have knowledge of a usable tool, or even can supply a link to a free tool (including script if required) to extract pertinent info, I'd be very grateful.
Whether an option postponing the start of a recording has any meaningful effect is unknown.
Regards and remember to have a bit of fun every day