I tried resolution by Jorge, it worked in one pcb but not in others.
With trial and luck, I finally figured it out, how to perfectly tent vias in Fusion 360
The PCBs which were tenting properly were having the following settings:
Via > Properties > Solder Mask > Off
Those which had it Auto were not working despite Layer Stack settings.
Conclusion:
(1) In fusion 360 under RULES Menu
Select Layer Stack (Design Preferences)> Masks
Set as follows:
Solder Mask 0 100 2
Stencil 0 0 0
Limit 14.5mil
(2) Under RULES menu > Design Rules
Drill size = 13.78mil
(3) All the Vias which are to be masked must have the following settings:
Via > Properties > Solder Mask > Off
Drill Size < 14.5mil, default you have set to 13.78mil above in (2)
Cross check those vias which do not appear tented especially their drill size and Solder Mask