The problem was: two instances of one object had the same frame and weren't responding.
I worked with two GM opened side by side, created a new room from scratch and added items, codes, one after the other, then made the room with the bug. Things were ok. It's when the second level was added that the bug returned in the buggy level; which was a duplicate of the second level.
I've read on the forum that coding with "instance_id" wasn't good; if something changes, the id will change and it won't be recognized. I did verify and everything seemed ok in the room description. After weeks on this, I've renamed the instances id to make it easier to refer to them (inst_LL, inst_RR); GM was fine about it but no success.
Today, at last, after all that full time searching, I've added in GUI:
Code:
if (instance_exists(obj))
{draw_text(123, 456, "obj_index = " + string (obj.image_index));}
If one variable becomes "true", the "image_index" changes. The GUI displayed "draw_text" in the second level but, surprise, not in the last one despite the two instances are on screen. I thought, maybe it could be the instances id, but each instance id stayed the same in the duplicated room.
When I clicked, in room editor, the option to change the instances names in the buggy level, GM now suddenly woke up "instances names must be unique" (I didn't add anything, just clicked the option). I changed the names (inst_LL, inst_RR for inst_LL2, inst_RR2), edited the codes; and now it's fine.
I've opened an older version of the project, where I haven't modified the instances names (the id are the same in the duplicated room) and edited; GM didn't reacted; launched the game and it was good.
I've played the whole project, all good. I do use "check resource names" and "check scripts"; I don't know if it verifies instance names but was always good. If this is a bug, I hope this can help for GM2.