With a direct connection like that the solenoid is probably pulling 40 - 50 Amps but just for a split second.
HOWEVER if the main lead to the solenoid is loose or has a build up of lead oxide on the battery terminal the 20 Amp fuse will blow in seconds because the split second of 40 - 45 Amps doesn't drop back down to 10 amps holding current as it should. (Look at the note against the wire from battery + to solenoid on the sketch below)
I suggest you remove the battery leads, scrape the terminals to show shiny metal then apply some Vaseline to prevent the Oxide build up.
Lead Oxide is an insulator, it can disconnect the battery lead even though the bolt is tight.
Most Guzzis have a 15 Amp fuse fitted but I believe a 20 is more appropriate for the high inrush current
Here's a sketch I did years ago to explain the current inrush that causes Status Interuptus

Note: This shows there are 2 coils in the solenoid, None of the Guzzi Schematics show that, The Bosch starters and the Chinese ones fitted to V7s also have 2 coils, I'm sure this also applies to the 1400 Californias.