To reproduce:
1. Download and import Unity Projects: 2D Platformer
2. Let two minions push you from different sides (as shown in the attached screenshot)
3. Notice that when one minion pushes you from the left side and another from the right side, the player does not take any damage

Won't fix because:
This is not a 2D physics bug, it's how the logic of the demo is written.

If you look at how player damage is dealt with in the "PlayerHealth" script you'll see that its based upon "OnCollisionEnter2D" so if the enemy continuous to stay in contact then it won't produce any more damage.

To counter this, the developers of this made it so that the player jumps away however when two enemy are on either side then it won't jump away.

A potential alternate way to deal damage here is to use "OnCollisionEnter2D" and "OnCollisionStay2D" so you continue to get damage if you stay in contact.

I believe the Learn team did this because when it was written, there was a problem with using OnCollisionStay2D as this wasn't being emitted when the Hero changed direction (change of scale) however that has been fixed in 4.5