Percona-server-5.6.41 and sql_mode
Found: another MySQL config is /usr/my.cnf and it contains:

sql_mode=NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES

so, setting sql_mode="" did the thing I need.

Debian stretch. Percona server was installed after removing installed MySQL-5.6 community. 

Intrecting thing: log doesn't mention this config for reading during server's startup...