It is not linked to the form from which the contact mechanism is created but to the contact mechanism form itself.
The problem is that the hidden widget for each type of mechanism has now an HTML validity check (e.g. url, email etc. must be valid) because we added a type attribute (issue9522). And the _change_value fill all of the fields with the value.
So if any of the hidden widget is invalid, the form is not validate but as the input is hidden the error message is not shown.
I think we should remove the type constraint on the URL widgets when they are hidden.

Behaviour found only for sao

On 6.0, from sale you can select a mechanism created previously from party, but it's not possible to create a mechanism from the Many2One window, because save button don't appear to work. Any warning raised or console message.
On 5.8 all works as expected.