Fergus Hammon - picking up thread from Cined.com - please see documented issue:
Can confirm this persists in the newest Premiere release 23.0.0 (Build 63).
This is what engineering will typically say: The issue is you're using the effects backwards. Interpret footage was NOT designed to be used as you are using it. Speed/Duration was.
So the proper effect for clip speed changes such as slo-mo workflows ... is the Speed/Duration effect. Which will work correctly with proxies.
'Interpret' was designed for changing from say one 'cadence' to another. Period. As in, going from 29.97 fps to 24 fps. Which is a more complicated and very different process.
Thank you Neil. I have a follow-up question and have conflicting experiences with offspeed media then. I'm cutting a project with Alexa LF footage. 23.976fps. Some shot at 40fps. Imported via Media Browser. 40fps footage reads and interprets as 23.976fps upon import. Why are there inconsistent offspeed workflows that are dictated by the camera system and metadata they each do or do not provide?
Was the camera setup using a "project" speed in-cam at 23.976, with a segment recorded at higher speed for slo-mo use?
Which is what my BMPCC4K does. I set a 'project' frame rate, and in the menus also set the framerate for off-speed shooting, which is normally faster so that when played back at the project-set speed, it's in slo-mo. Premiere auto-sets that to the same speed on playback as the project speed. So the visual result is slow motion.
Which is a typical slo-mo process in pro cams.
Is that what you're seeing?