I just found the issue is related to the session lenght!
To do this job I usually create 23:59:59:00 long sessions to then import the files at their original Time Code.
I was working on a shorter session today and I could zoom in fully.
So after few tests I found that when sessions are longer than 12 hours, the zoom in won’t work.
If the session lenght is up to 12:00:00:00 it will work. Longer than that won’t.
How weird, but at list I found what it is.
Is still an annoyance for long days on set, when the working hours are longer than 12. :-/