After updating to Gutenberg 10.2.1 I got a load of Warnings and also unexpected HTML output.
There was no problem with Gutenberg 10.1.1
I can't see from the change log what caused this.
Time to investigate my code.

As an aside, I also had broken images... but this was down to not having an https connection in my local server.
Normally I'd see Kaspersky having a moan about insecure connections.