So I've duplicated this question to Apple support forum and it looks like it is current known behaviour. https://developer.apple.com/forums/thread/779223

As for other app as far as I understand they use share extensions to open this kind of file. And ones that I thought don't use share extension actually use it but without proper UI and invoking opening main app using objective C runtime to overpass extension limitations that is actually looks like a bad way to use extension. Some of the ways to do this may be found here. Also I've tried sample app with Action extension it works with p12 as well.