Another Tunecore request not to embed metadata in files, with warnings that it will likely cause a delivery issue. The post is almost five years old, but they still recommend not embedding any metadata (which would include ISRC) on their support pages.
And further confirmation that the stores do the conversions to lossy, not the aggregators:
Q: Do the digital stores do the conversion to mp3 or does TuneCore?
A: The stores convert on their end, while TuneCore is required to send the files in WAV format.
https://www.tunecore.com/blog/2013/09/how-to-get-your-audio-files-ready-for-distribution.html
And similar metadata information from CD Baby:
So I really have to assume that the ISRC embedded in my wav file is probably not going to be read or relied on as being the final word for being the correct ISRC for the track, after everything is released. I can only assume that the final ISRC is transmitted separately.