When specifying a system identity with a singe word, for example: router, it does not show up in remote syslogs. But when I type an identity that contains one space, example: Firstname Lastname the "Lastname" shows up in the remote syslog.

Is this a feature or a bug?

Example 1 from syslog-ng: identity is set to "Uto Logi":
Jun 15 10:54:53 192.168.10.10 Logi [uto_logi]: defconf assigned 192.168.88.219 to B4:82:FE:E5:93:7B

Example 2 from syslog-ng: identity set to "Uto":
Jun 15 10:55:55 192.168.10.10 [uto_logi]: defconf assigned 192.168.88.219 to B4:82:FE:E5:93:7B

Are you sure that is the problem of Mikrotik ? Just asking ... no offence. You are showing logs from syslog-ng and maybe it is problem of syslog-ng.
Have you checked non Mikrotik devices with two-word names ?

I use rsyslog and it works OK for me, both for single word and multi word names (all words are shown).
Did you use the BSD syslog option?

You are right that it might be also the syslog-ng cutting out the first part of the name/identity. I'm unable to debug that.

When unchecking BSD option the indentity is not shown at all on syslog.

Try it with rsyslog, e.g. a Raspberry Pi, to see if that works...

Works for me with Syslog-NG out of the box, I'm sending traffic via IPv6 with BSD-Syslog option. It looks like this in my NMS:

Changed to rsyslog and now it's working fine. Sorry and thank you