Okay, so I finally got the ruleproc debug logs.
My bad, I enabled ruleproc_debug=1 in /etc/gromox/exmdb_local.cfg instead of /etc/gromox/gromox.cfg.
Really a keyboard-chair interface problem ^^
I created a fresh "TEST-RULE" in grommunio-web, which moves messages from surname.familyname@mydomain.tld to folder "TEST" in account othersurname.otherfamilyname@mydomain.tld
Now when sending a message from surname.familyname@mydomain.tld to othersurname.otherfamilyname@mydomain.tld , the rule doesn't get executed and I get the following log (username and domain properly redacted to reflect the above rule while keeping case and exact syntax):
juil. 30 09:51:34 mail.mydomain.tld gromox-delivery[1355]: Rule_Condition RES_COMMENT{props[4]={PT_LONG{1/0x1},PT_BINARY{[116]="EX:/O=I6509FB5C/OU=EXCHANGE ADMINISTRATIVE GROUP (FYDIBOHF23SPDLT)/CN=RECIPIENTS/CN=0100000001000000-SURNAME.FAMILYNAME^@"},PT_UNICODE{[46]="Surname Familyname <surname.familyname@mydomain.tld>"},PT_LONG{0/0x0},},res={RES_PROP{val(c1d0102h) == PT_BINARY{[116]="EX:/O=I6509FB5C/OU=EXCHANGE ADMINISTRATIVE GROUP (FYDIBOHF23SPDLT)/CN=RECIPIENTS/CN=0100000001000000-SURNAME.FAMILYNAME^@"}}}}
Perhaps I will post this on github, I have no clue what to do with that debug log, except noticing that the comparaison lacks PT_UNICODE property on one side ^^