The script of the previous post may solve the inconsistency explained at the beginning of the thread, but it has its drawbacks:
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Not a neat solution. In fact, I will wait a couple of day before considering the thread solved to see if somebody finds a problem I have missed
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LightDM Unlocking fails randomly (at least in my PC). Please see User gets logged out after unlocking! - LightDM/Plasma(X11)