Unfortunately, remmina is completely unusable in the following scenario:

I have an RDP server I need to connect to via SSH tunneling. This looks
like the following in ~/.ssh/config:

Host mycompanyjumphost
 Hostname xxx

Host customerjumphost
 Hostname yyy
 ProxyJump mycompanyjumphost

The RDP server is host zzz and only reachable by 'ssh yyy', that is,
'ssh yyy "nc zzz 3389"' will connect to it.

If I set up a Remmina profile where I put “zzz” into “Server” and
“yyy” into “Custom” under “SSH Tunnel”, it complains that it cannot
connect to yyy port 22. Of course it cannot because it’s proxied.

Please fix this.

Unfortunately, you uploaded a new version of remmina to unstable
during the freeze against the rules, so fixing this in trixie will
be hard…

Can confirm this issue as well (and more):

I recently migrated my system from OpenSuse 15.6 to trixie, and the
config containing a ProxyJump entry worked fine there
(remmina-1.4.35-150600.1.2.x86_64, libssh4-0.9.8-150600.11.6.1.x86_64).
Under trixie, I had to switch to a ProxyCommand workaround to make it
work again.

Given the different versions I used before vs. what trixie provides now,
maybe it is some upstream regression. And there is indeed
https://gitlab.com/Remmina/Remmina/-/issues/3426 which I just commented on.