I am trying to add Metafields to a newly created Product Image through either the privateMetafieldUpsert or the metafieldsSet mutations. The privateMetafieldUpsert mutation returns a "the owner record does not exist" error on the "input" field while ...

You bet. I was going crazy trying to figure it out too. My first batch would work, then my second batch would fail. Every time... Thanks to Mcepo I was able to figure the bug out!

I am seeing the same issue, HOWEVER, my JSONL is registered if the first item is under 5000 characters, regardless of the length of consecutive items.