I can't find any specs that say it, but Exif 0x9999 seems to always contain a json value.
Most are from Xiaomi cameras, but I've also seen a few others using it.
Examples:
https://fotoforensics.com/analysis.php?id=0072b0f47e7e1c4c382f986e25177f2b6b833c08.1835687&fmt=orig
https://fotoforensics.com/analysis.php?id=02fe28c1627f780d777460b3ec2969658e6dad76.4516353&fmt=orig
Thanks. Yes, I've seen this. But now that you mention it, maybe it would make sense to extract the JSON values. I'll think about this.
- Phil