To start this off, I wanna mention that this happened on my realm world.

1st - This world has 2 portals in overworld and 2 in nether. 3 of them are pretty standard size and the one which is problematic - 11x11. This big portal is very strange - if I am on it's bottom - it teleports me to first nether portal, but if I try to crouch near it's top - it teleports me to second portal. 

2nd - I probably would have never written this report if this not happened. But I was as always trying to teleport from the overworld to the 2nd nether portal and suddenly it teleported me... well, idk where, because I saw just black screen and heard only sounds of suffocation... after I died I found my EXP at the bottom of the big portal but my items were nowhere to be found... Not near any of nether portals and even not at the same X, Y, Z in the nether as overworld...

I hope this helps you and sorry for my bad English! ;D