Verify Rejoin is PUT not PATCH
Overview
Here we verify that when join is called for a second time, any fields not set in the object in the second join are not preserved.
Conclusion
Sadly (for ad tech) this is the case.
Test and Results
We join an IG with real bidding logic and ads, run an auction and observe it win below; then we re-join but with only the owner/name/lifetime, re-run the auction and observe a null value below.
Live Test Results
IG Action Sequence from Dev Tools
If you load this page while watching the Dev Tools --> Application --> Storage --> Interest Groups panel, you can see the IG join the first time, bid, and then win, followed by the "limited rejoin" and load for the second auction, but then no second bid (or win) because it no longer has any bidding js, signals, etc (you can see the expected sequence and verification of the IG state in the image below).