Propably its one of font problem in linux client. It's most know problem never fixed, cause cipsoft dont support linux client.
There are some fix, but never work for me. But u can try it. Just type in google "tibia ubuntu font problem"
There are some threads
https://www.reddit.com/r/TibiaMMO/comments/5nz1mp/tibia_11_on_linux/
http://tibiacommunity.com/linux-tibia-11-font-rendering-problem
Or, maybe linux are blocking this key shortcut, like linux is blocking alt key.