Maven not dropping the Invitation?

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Jatin_GGG wrote:

Thanks for your report, the Invitation for 10 bosses is NOT guaranteed to drop but instead has a chance to drop.

We are changing how this works in the upcoming patch, the Patch Preview can be found here.

The upcoming change will guarantee that the Invitation will drop for that Region when you collect the 10th map boss.
After the first time a guaranteed Invitation has dropped in a Region, it will no longer be guaranteed and will only have a chance to drop (back to how it currently is).