TNT blows up twice when it falls 74 blocks from the sky into and end portal

if you build up 74 blocks above an end portal and drop a TNT from the side into the portal, it will blow up at the portal. For some reason the TNT explosion at the portal does not damage the blocks around and does not ignite other TNT if you have it nearby. Once you enter the portal, TNT will blow up and will cause a hole.

It can only be 74 blocks. not 73- or 75+