Just add a line:

ForceCommand /bin/bash

to /etc/ssh/sshd_config

Or whatever shell is needed, which exists on the system.

Item will get unsupported with error logged:

Cannot read data from SSH server: timeout error

The error logged according to this:
https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/src/libs/zbxpoller/ssh2_run.c#491

"man sshd_config" for the option:

ForceCommand
Forces the execution of the command specified by ForceCommand, ignoring any command supplied by the client and ~/.ssh/rc if present. The command is invoked by using the user's login shell with the -c option. This applies to shell, command, or subsystem execution. It is most useful inside a Match block. The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment
variable. Specifying a command of internal-sftp will force the use of an in-process SFTP server that requires no support files when used with ChrootDirectory. The default is none.

I've played with the option (also trying "/bin/sh" value) and see a real influence how processes looking, here is "sh" at last line:

# pstree -a | grep sshd -A4
|-sshd
|   `-sshd-session
|       `-sshd-session
|           `-sh

When I tried with "/bin/bash", then processes structures tree is identical to when the parameter is not defined at all.

Interactively I do not see much difference except of "Welcome ...." block is presented if the parameter is not defined.
But last invitation line after log-in is always the same for bash:

username@hostname:~$

Looking to https://libssh2.org/libssh2_channel_read_ex.html I feel it might be related to a "channel" parameter of the function call.

TCP/SSH session is happening of course. I can just see a differences in number of TCP packets.
Success session with command "echo 5" has 23 packets, while the failing one has 18 packets.