Thanks for looking into that and @ronau thanks for reporting this in detail.
I think I mentioned the same issue a while ago thinking I would get into it shortly after. Unfortunately a black hole appeared and it’s eating all my time!
My workaround is to kill the front-end Caddy (caddy stop does not work as it looks like it is waiting for all connections to close), stop the back-end caddy and delete the certificate folder. Then restarting both Caddy end and have new certificates issued.