The rules are absolutely clear.
I think you did the right thing, not to report the commander but look for further knowledge elsewhere first and only then make your OWN opinion. Later when you do not feel the need to refer to yourself as "junior" (I do call junior myself) we will grow to be experienced for such decisions. A report may stay in captains papers and yours(!) for a long time - words are mighty, let's tread lightly.
If the captain was a really nice person, while seeking guidance from your CC superior, perhaps it may even be possible to approach him and ask for help with your dilemma. Now, that would be the ideal case.
I like the name of the forum you chose, the door is a security feature. So what are the effects on:
Security?
Safety?
CRM?
Should any unforeseen circumstances arise in flight (bad weather, technical problem, abusive drunks, major security issue), the key to keeping all souls on board happy and unharmed is mutual trust. If one can evolve this to mutual understanding, even better.
To me it seems that you faced a situation where the trust (from your point of view) was rightfully questioned. I suggest you endeavour not to lose the trust but try to find understanding instead.
All said, the rules for security doors are straight and leave NO room for mis-interpretation.
261A:
To close and LOCK the door is SOP. S stands for standard.
Something went unusual and the crew returned to their routine just after the point where you lock and check the door? Finger trouble? I do not know. I remember that once the door remained unlocked, flung open with loud bang on rotation just to be slammed shut two or three times VERY shortly afterwards. The captain did make his opinion on this very clear later.
Cheers,
FD
(the-unreal)