Technically, a 1-sec GOP would be encoder Level 4.1, but changing encoder levels didn't make a difference to the results above.
I've never had great luck with TMPGEnc Authoring Works accepting video which uses encoder Level 4.1. They say it's Blu-ray compatible, but I never found how to make it work. AW always flags the 4.1 tracks as incompatible with Blu-ray, by reason of encoder level. So I just use encoder Level 4.0, which works fine. TMPGEnc Authoring Works accepts either 1-sec or 2-sec GOPs with Level 4.0 (which should be 2-sec GOP only, according to spec).
Lol, and as mentioned . . . TECHNICALLY, 1080p29.97 is non-compliant with Blu-ray anyway.
In any case, all test files above were rendered using Voukoder, and therefore FFmpeg. And they all used NVENC on an Ampere card. None of my tests yielded a file with an extra leading I-frame, so this is quite clearly neither an FFmpeg issue, nor an NVENC issue.
can be produced directly using FFMEG
If you are getting this problem even while using command-line FFmpeg, then we can also rule out Voukoder and its connector as the cause.
If the error occurs in command-line FFmpeg across multiple framerates and encoders (you've mentioned both NVENC and x264), then there must be a problem with your encoding settings, or maybe even with your source file's encoding from Premiere.
Out of curiosity, what are your Voukoder encode settings? And what exact command-line are you using for your FFmpeg encode?