I believe it depends on the engine, as normally they would need to support a vast number of devices, so they tend to cover the versions with more usage.
Your previous information was very valuable, as it led me to explore the code of the exporting Android target and I found out that indeed, the minimal SDK in the code is 21, so the default setting on the build is not matching with the actual code on the export.
But I can rebuild a custom export with the tweaked minimal SDK set to 24, so I’ll try that. Thank you for the test offer, I may take you up on that.