HSv3 throws many "Tried connecting to router at [IP:port], but RSA identity key was not as expected"

A popular-ish HSv3 operator contacted me and told me that they've been getting lots of warnings on their logs:

[warn] Tried connecting to router at [IP:port], but RSA identity key was not as expected: wanted [hex string] + [base64 string] but got [same hex string] + no ed25519 key.

They are afraid it's some sort of downgrade attack. We should look into this.