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That is correct this is by design. If one of the testers wants to get a non-flight package they would need to be removed from any flight groups they are currently in.
It kind of says this in the docs below you could pass along to the partner.
Package flights - UWP applications | Microsoft Learn
Anyone who has not been added to one of your flight groups, or is using a device that doesn’t support package flights, will get packages from the non-flighted submission.
You can use package flights to distribute specific packages to a limited group of testers. The packages you've already published to the Store will be used for your other customers, so their experience won't be disrupted.
Thank you.
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