Upserting to <7.0 clusters does not upsert the content

Running an upsert on the DefaultCollection does not upsert the content with the document for <7.0 server version, it just upserts an empty doc ( "{}" in the docs UI)

It can be recreated with just a basic upsert:
await Bucket.DefaultCollection.UpsertAsync("id", "content");

The upsert will succeed, but if you inspect the doc there is no content in the doc.

This happens with the current head of master (798e6e34c6180d339c53076fbf4cc2547c81d496) and appears not to have worked since NCBC-2839 (c410926a8131eb40483cd60370561dc0fcbe6244) and the previous commit.

Here are some logs of this happening, though there doesn't appear to be any issues reported. SdkdConsole.log.zip