The rule is that a gliss lasts for the duration of the note from which it comes.
When a gliss emanates from a tied note, Dorico automatically selects both note heads of any starting note, and then inserts the gliss from the end point. This is not necessarily right. E.g. this:
…might more properly appear like this, with the gliss starting from the red arrow in the above:
I know about this:
…but is there a way in Dorico of specifying from the get-go, from wherence the gliss should emanate, when this is as part of two or more tied notes?