Netherportal blocked by items

If you build a netportal and you have a block in the netportal and then try to light the portal, this will not work even if you remove the block and the fire in the netportal burns in all specified versions. This also applies to flowing water and lava.

It may be that this is not a bug but it would be better if the fire in the portal was burning no matter if there was a block in the portal before or not that then the Netherportal is still on.