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For me, the problem was in one of the frameworks that I had added to my workspace and not in my primary project. Specifically, I was using the Split.io framework version 2.13.0. Looking at the Build Phases of the framework, the sequence was Dependencies Compile Sources Link Binary With Libraries Headers Copy Bundle Resources Changing it to have the Headers step earlier, like this, fixed the problem for me. Dependencies Headers Compile Sources Link Binary With Libraries Copy Bundle Resources This problem is also fixed in Split framework version 2.16.2. Updating to this latest release is probably the easiest way to fix the problem.

I just spoke with someone from the App launching and linking lab. At a high level, this is what I learned. the documentation, etc isn't done yet, but they are working on it there isn't a specific date when they will have the documentation completed. But certainly before iOS 14 release. there is no session like some other sensitive features, you have to get a special entitlement in order to be able to do this send an email to default-app-requests@apple.com to have them review your app and grant you the entitlement they will review your app to make sure that you are actually an email client, can send/receive emails and not a game, etc your app can't have both the email client and web browser entitlement, you can do one or the other the setting to switch which app is the default mailto handler doesn't show up until there is more than one choice on the system the setting appears under the settings for the 3rd party app there is a runtime check for the presence of the entitlement and that mailto: is handled the mail client entitlement is set in the plist as com.apple.developer.mail-client=true the web browser entitlement is set in the plist as com.apple.developer.web-browser=true the web browser has to be able to handle both http and https requests, and a few other things this should be possible in the current beta 1 code If you have more questions, the Safari or Webkit labs might be able to help

https://developer.apple.com/forums/thread/650027 has the answer

I just spoke with someone from the App launching and linking lab. At a high level, this is what I learned. the documentation, etc isn't done yet, but they are working on it there isn't a specific date when they will have the documentation completed. But certainly before iOS 14 release. there is no session like some other sensitive features, you have to get a special entitlement in order to be able to do this send an email to default-app-requests@apple.com to have them review your app and grant you the entitlement they will review your app to make sure that you are actually an email client, can send/receive emails and not a game, etc your app can't have both the email client and web browser entitlement, you can do one or the other the setting to switch which app is the default mailto handler doesn't show up until there is more than one choice on the system the setting appears under the settings for the 3rd party app there is a runtime check for the presence of the entitlement and that mailto: is handled the mail client entitlement is set in the plist as com.apple.developer.mail-client=true the web browser entitlement is set in the plist as com.apple.developer.web-browser=true the web browser has to be able to handle both http and https requests, and a few other things this should be possible in the current beta 1 code If you have more questions, the Safari or Webkit labs might be able to help