I have just followed your points to the letter.Vetrogor said:1. Rebell from Fierdsvein
2. Conqer capital Javicsholm
3. Ask for protection to Fierdsvein and become a vassal
4. Lords spawn at Javicsholm and gathering troops
5. Rebelled from Fierdsvein
Lords are siting at Javicsholm and don't come out. They say "enemies_are_reported_to_be_nearby_and_we_should_stand_ready_to_either_man_the_walls_or_sortie_out_to_do_battle"
Both lords who entered Javiksholm with their armies, as well as lords who had spawned there after lost battles, left the town as soon as they had enough troops.
One of the lords had no money at all and even him eventually left the town after a month (thanks to the "minimal weekly allowance").
I can't recreate described behaviour based on your points.
Who is their enemy? AI armies stand next to the city all the time? You must be friend as you can talk to the lords in the castle.
If described situation is real then all lords from the given kingdom would eventually be defeated and "locked" in the capital. Something like that would be rather hard to miss as it is game breaking bug, but I haven't seen any reports of this kind.
EDIT1: By "rebell" in points 1 and 5 do you mean:
1. speak with your king (and as a result Javiksholm is passed to Fierdsvain and has no owner)
or
2. conquer new fief, ask it for yourself, king refuses, you leave with your fiefs (and as a result Javiksholm is yours)?
EDIT2: I have checked both of above options and all lords left castle when they recruited enough soldiers.