The table `m` is MyISAM but, as far as I can tell, no warning occurs with other storage engines. The table `t` is CONNECT, which seems to be the only engine that issues a warning.
I report this behavior because you probably want to be aware about inconsistencies between engines. But, to be honest, I prefer CONNECT's behavior: when a trx starts, table locks are lost, and this is something I want to know. I hope that a more informative warning will be issued, no matter which engine I am using.