The bug:

Dispensers can't light a Nether Portal if the fire doesn't have a supporting block.

Steps to Reproduce:

1. Build a portal big enough to have an air block without any blocks on its surroundings. Minimum size: 5x5 frame;
2. Place a dispenser with a flint and steel behind the unsupported air block;
3. Activate it.

Observed Results:
The portal doesn't light up.

Expected Results:
The portal gets lit.

Screenshots/Videos attached: Yes

Notes:

Original description:

A nether portal don't light up when I activate a dispenser which is not against the side or the bottom obsidian. It work when you have a flammable block next to it (as shown in the seconde screen).

Same for the flint and steel, it work when you're next to the obsidian (not the top one) with all blocks but in the middle it do not light up the portal if it's not a flammable block.

Although it work with the fire-charge on all blocks. You can't use a fire-charges in a dispenser, obviously, becauseit shoot it ! (it's entended, i know)

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What expected :

In all the previous versions it worked in all position, and all the blocks whether you use a dispenser, or the flint and steel directly with the hand.

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Consecutive to :

In the 1.13 update the ability to create a flame with a flint and steel on all blocks even if there is no block bellow was remove.

This explains why you can't light up the portal with a flint and steel and when it's not against the obsidian, because you can't activate the flint and steel.
This do not explains why you can create a flame on a none flammable block when it's next to to the obsidian inside the portal frame (don't create a flame when it's not inside).