RouterOS DHCP Client $(hostname) variable contains white space characters from RouterOS Identity making it incompatible with any DHCP servers and their “dynamic DNS update” (not to be confused with DDNS) which don’t clean names.rfc2181 rfc2132 rfc4702
And no, Mikrotik, nobody here is interested whose fault it is, just that it is not working / compatible as expected in comparison to common practice. Just strip the non-FQDN allowed characters and trim the length, when both the sending client does this and the receiving dhcp server does this the world becomes a slightly better place in every environment. If you don’t like it, make cleaning optional, the users still need it as a Identity which is literally a “64 character string”.
p.s. Identity (64 characters) is also used for SSID which trims at 32 characters