Portalblocks 90 and 119 should disappear when placing manually

(After reading two comments I realized my description was not clear enough. So I changed it to make it more clear)

Background
In creative and survival mode you can use portalblock 90 (Nether) and 119 (End) and place it manually. If you do so, they show a specific behavior:
1) If you try to construct a nether portal manually by placing TWO nether portalblocks together, they disappear. Apparently it shouldn't be possible to create a portal manually.
2) If you try to construct an end portal manually by placing an end portalblock in the End or the Nether, it disappears. Apparently it shouldn't be possible to do this too.

What I expected to happen was...:
1) ... that placing a SINGLE nether portalblock somewhere, it would disappear too.
2) ... that placing an end portalblock in the OVERWORLD it would disappear too.

What actually happened was...:
1) Placing a SINGLE nether portalblock (block id 90) somewhere, it stays. This is possible in Overworld, Nether and even in The End (where you should NEVER be able to create a nether portal). And surprisingly you can go through this single block portal into the Nether. (Side effect: If you go through without an existing normal Nether Portal, you arrive in an invisible Nether. The Nether is not visible until logging out and in).
2) Placing an end portalblock (block id 119) in the Overworld manually, it stays instead of immediately disappearing.

Summary
Apparently it shouldn't be possible to create a nether portal or an end portal manually. But this is not implemented consequently.
1) Two nether portalblocks disappear - one nether portalblock not. Disappears with block updates.
2) An end portalblock disappears in the End and in the Nether. In the Overworld it doesn't disappear.