Guys, thanks for the replies. Your answers have pointed some possibilities:

* Neither I or my mechanic faced the bottom bracket shell. Maybe this could solve this riddle, but don’t know any wrench who does this in my area. Let’s see if I can find one...

* Crankycrank has mentioned the axle might come loose from the drive side. In fact, it’s coming loose from the non drive side, where the left arm holds it on the spindle. The Allen screws are correctly torqued, should it worth the risk of applying more torque on them?

* Also, the crank has been inspected: no ovalization, just a slight mark on the bearing contact points, barely visible, not detected on the digital caliper (0.05mm precision). Maybe it’s detectable on a micrometer, but if it’s that sensitive it shouldn’t last as long as other fellows report. I am right assuming this?

* About the small plastic tab with a pin, the one who gets into a hole on the spindle, on the non drive side (I guess I can’t describe it better than this): it doesn’t even bent, despite the fact the non drive side arm sliding out. The hole is somewhat large for the pin, so maybe there’s some tolerances allowable. Is it really like that?

Thanks in advance for any answers!