When me and my brother were playing together, split-screen, I build a nether portal, after my brother already had one. And when I entered it, I died almost instantly suffering a few suffocation hits, before me dying instantly and it saying I died by a creeper. When I went back threw the portal, there was NO creeper damage or items scattered.

The portal was freshly made, during the day, so creepers could not have travelled threw it and it was far enough away where it did not link to the other portal.