I don't want this to degenerate into rules lawyering but I do want to answer adequately.
Mandates often are designed to override typical rule-sets and that's no different here.
(Ex. Factions can normally invade for two hexes taking the path of least resistance back to the main cloud but there are mandates (Total War) that provide as many as possible 6 hex gains or losses. Take Relentless Horde as well...It allows that faction to invade two simultaneously.)
So an example would be say if the GA decided it was going to invade the TSO. The TSO, having vassal as it's mandate couls then say, OK you are invading us. So we are going to turn three of your core hexes into a TSO vassal.
Because the rules state a faction can only initiate one invasion at a time, all the GA can do is so, OK they are all yours. I guess our PDFs were napping.
That's not going to be a penalty imposed.
It would be
wild of us to do that when we don't do it with
any other mandate that alters the standard rules.
Just like I stated above, mandates deliberately supersede the standard rule set and allow factions to do things that they otherwise wouldn't be able to do. Is it currently possible that two factions with Vassal States could land in the same target at the same time? Is it possible that the defender might need to deal with this from more than one source? Yes. Unlikely, but yes. It's hampered by a 60 day cooldown and the general consensus of the community that we can tell engaging stories without beating up on each other. It's built into the mandate that the defending faction has the option to block the claim. We're not taking that away just like we wouldn't suddenly tell a faction they can only have 2 hexes (standard) when their mandate gives them 6.
Then there is the wording, it's the order of the sentences
When claiming space occupied by another Major an Invasion can be launched in an attempt to block the claim. If this faction fails they lose 2 Hexes.
Trigger - vassal claim made
Action - faction declares invasion to defend
Result - faction fails (in their invasion)
Punishment - loses 2 hexes (they are already losing 3 that turn into a vassal)
There's a lot of mental gymnastics here taking a
strong leap in the wrong direction, however, we're already working on updating the wording a little. I'm a fan of things making sense...And if it doesn't a look-over doesn't hurt.
If anyone has questions about the intention in the interim, etc, feel free to ask.