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I opened a ticket in WordPress’ Stackexchange ) but so far it seems there’s no solution: I need to add both URL parameters and an #anchor in the custom link, and for the theme’s way of working I need the anchor after the URL parameters. But it seems that when they are both in the URL, the parameters get cut.
Is it some default functionality of WordPress?

Your example from StackExchange is working fine for me [https://wp1.samuelezappala.it/?tab=prestito-e-raccolta-libri#servizi])
Or are you saying it doesn’t working when you put the link in a menu? I’m sure I’ve done this in the past without any problems. Is the URL loading correctly in the source code?