How can I check my config files my.cnf for example

I have 2 config files, master.cnf and slave.cnf, each containing a group [mysdql0] or [mysqld1] respectively.

I have an almost identical setup across different servers and all but one work correctly with a single my.cnf file which has the following lines;

!include /etc/mysql/master.cnf
!include /etc/mysal/slave.cnf

and when I run mysqld_multi report I get the output I expect, relating to mysqld0 & mysqld1

However, there is one box that simply returns no instance groups and in an attempt to diagnose this issue, I moved the configuration for master.cnf into my.cnf and to my surprise I am then able to run mysqld_multi report and also start and stop that instance without issue.

For whatever reason, when I split this out into the two files as mentioned above and a single my.cnf file, it just doesnt see any instance groups.

My question is twofold reallyl

1) What could be causing this? given the same configuration is up and running on a few other servers.

2) Secondly, how can I diagnose this issue? (is there a way to see if the my.cnf file is being parsed and more importantly getting to the !include lines etc?

I’ve tried running mysqld –validate-config on newer versions but this just gives an error unknown option –validate-config

I’m running on a mixture of Debian boxes and version range including;

5.5.55-0+deb8u1 and 10.3.27-MariaDB-0+deb10u1

UPDATE 21-JAN-2021
I’ve updated my post based on comments received and focusing on the prolbem box only. the above can be treated as merely background information.

Ok, so I have amended things slightly in an attempt to put across what the current problem is.

So currently, I have a file /etc/mysql/my.cnf which is as follows;

[client-server]

!include /etc/mysql/my_master.cnf
!include /etc/mysql/my_slave.cnf

I also have two other cnf files, surprisingly my_master.cnf and also my_slave.cnf and these contain sections as below;

my_master.cnf contains [mysqld0]
my_slave.cnf contains [mysqld1]

both of the above have configurations that work.

If I run mysqld_multi report I get;

Reporting MySQL servers
No groups to be reported (check your GNRs)

If I run my_print_defaults –config-file=/etc/mysql/my.cnf mysqld0

I get the output I’m expecting for [mysqld0]

and similarly, if I run my_print_defaults –config-file=/etc/mysql/my.cnf mysqld1

I get the output I’m expecting for [mysqld1]

but still can neither report, nor start either of my instances with mysqld_multi

If I simply rename my_master.cnf or my_slave.cnf to my.cnf then mysqld_report reports as expected and I can also start and stop that instance as expected.