This is most likely an intentional feature, however /tban bans for 5 minutes even if a time is specified. /tempban bans for a time specified.
For example:
OP X has broken a rule repeatedly and I want to ban them for 10 minutes. I run /tban.
/tban X 10m Rule broken repeatedly
It will ban them for 5 minutes with the reason 10m Rule broken repeatedly. However, /tempban works as it should.