Export Frame JPEG "looks darker" (embedded sRGB color profile should be Rec.709)
Since 14.4.0, when using Export Frame to export a JPEG, it is darker than a PNG or TIFF export, while they used to be consistent.
This can easily be reproduced by adding a HD Bars and Tone in a fresh project/sequence and using Export Frame to export a JPEG and PNG still, and comparing them in Photoshop.
That "darker" output is exactly the difference between sRGB IEC61966-2.1 and Rec.709 Gamma 2.4, for example if you open the "good" PNG in Photoshop assign it a Rec.709 Gamma 2.4 profile (Edit => Assign Profile) it now looks like the JPEG. But Premiere embeds a sRGB IEC61966-2.1 profile in the JPEG, and this makes me think that it should actually be Rec.709 Gamma 2.4 (it would still look darker but it would be properly tagged).
To go a step further, we can replace the sRGB profile that Premiere embedded in the JPEG by a Rec.709 profile using ImageMagick (here on macOS, the command would need to be tweaked on Windows but I do have that bug on both macOS and Windows).
convert test.00000000.Still002.jpg -profile /Library/ColorSync/Profiles/Recommended/VideoHD.icc test.00000000.Still002-rec709.jpg
(VideoHD.icc is the ICC profile for Rec.709 Gamma 2.4).
Now if we import that new file in Photoshop, and assign it a sRGB IEC61966-2.1 profile (which we couldn't do before because Photoshop thought it was already sRGB), it looks consistent with the PNG.
I believe that the JPEG exported stills should be embedded with the Rec.709 profile because it seems that they contain Rec.709 data and not sRGB, but it would be even better if they were exported as actual sRGB like the PNG and TIFF files are, so that it would be even easier to work with them out of the box in Photoshop and other apps while keeping a consistent look.