the validation fails also after chosing the widget kind of "dropdown" and selecting "other:" option.

ok. I got it. There's also a flag inside the "Logic" pane who does set mandatory the other text field.
But the problem with the UI still exists because validation message "you must write something inside other text-field" flashes red when other-text-field isn't mandatory by logic.

Yes : right.

Can you provide a sample lss ? and a screenshot maybe ?

Well, never mind. -_-'
I can't reproduce it. Maybe we had some other misconfiguration inside that survey.
Issue can be cancelled. Sorry for misunderstanding.

The "sibiling" situation works and validates correctly, when you choose checkbox multiple answer list with mandatory and others options enabled, the validation blocks the partecipant if they don't type anything inside the "other" option.

You don't have red warning about other if other is not checked too ?

Not a major issue, but sill an issue ;)

648 - is the single radio question, with mandatory and others switched on
649 - is the multiple checkbox with mandatory and others switched on

When you tick "other" in single-radio, nothing happens
When you tick "other" in multiple, it focus the text-field

Single Radio Question has one more setting inside "Logic" Pane, who lets you force text-input digits
Multiple Checkbox Question does it by default. (you cannot simply refer to "other, you must enter something. for example the word other")

When you go next page, validation of single-radio asks to write some text (logic pane text-field is N, i expected you chose just other). Same for multiple checkbox.

We have different situation to handle in both questions:

A) other (with no text)
B) other (with mandatory text)

Screenshot_2021-02-09 Questo è anonimo(1).png (28,864 bytes)   

Screenshot_2021-02-09 Questo è anonimo(1).png (28,864 bytes)   

Screenshot_2021-02-09 Questo è anonimo.png (27,816 bytes)   

Screenshot_2021-02-09 Questo è anonimo.png (27,816 bytes)