This thread is about MAC address which is always uppercase.
Your question is about hostname which can be anything.
A little search with a possible solution, left to the reader to modify as needed:
Using this it doesn't matter anymore what case whatever parameter is. Not even MAC addresses 
[xyz@RB5009] > ip dhcp-server/lease/print where host-name~[$strfind ("PRO")]
Flags: D - DYNAMIC
Columns: ADDRESS, MAC-ADDRESS, HOST-NAME, SERVER, STATUS, LAST-SEEN
ADDRESS MAC-ADDRESS HOST-NAME SERVER STATUS LAST-SEEN
5 192.168.2.50 00:E0:4C:68:02:B3 PRO3-0004 VLAN2 bound 1h5m46s
[xyz@RB5009] > ip dhcp-server/lease/print where host-name~[$strfind ("pro")]
Flags: D - DYNAMIC
Columns: ADDRESS, MAC-ADDRESS, HOST-NAME, SERVER, STATUS, LAST-SEEN
ADDRESS MAC-ADDRESS HOST-NAME SERVER STATUS LAST-SEEN
5 192.168.2.50 00:E0:4C:68:02:B3 PRO3-0004 VLAN2 bound 1h5m50s