We had something similar happen with an Enterprise App. I'm not sure which one of the following steps or a combination of these steps resolved the issue, but it's now installing on iOS 15 beta.
Updated from Catalina to Big SUR
Updated from Xcode 12.4 to 12.5.1
Raise deployment target from iOS 13.2 to iOS 14.5
iamtron01@mac.com
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