[Kiwikkiwik]

OK so actually exactly the opposite is true. Fortifications are vulnerable to direct fire and invulnerable to indirect fire, shelling. The roof is always as thick as possible, say a metre of reinforced concrete, impervious to artillery and of course the chance of a direct hit is vanishingly small anyway. However direct fire can easily lob in through the embrasure.
A squad leaving a bunker should have good situational awareness the fortification is stationary and they will have drills in place. At the very least they had to get into it so probably saw something of the immediate surrounds on the way in. Anyway a 3 minute turn is enough time to be fully deployed by turns end.
Bunkers are specifically designed to mow down infantry WHILE the bunker is being shelled, that is their purpose. In the game they do not represent a difficulty because they can be suppressed into uselessness in a single turn as they get the same level of suppression as a squad lying in the open. Compounding this is the fact that suppression from indirect fire artillery is way too high by at least a factor of 5 which I demonstrated in the thread on feature requests. Suppression IS correctly very low for the carried troops while being shelled, except for some odd reason when they exit, when the bunkers suppression is added on to them and it skyrockets. To me that looks like a bug.

A suppression level of 1 or 2 would be appropriate for a unit leaving a shelled bunker because in the bunker they are aware that they are safe from shelling.
I would think it makes sense if the shelled carried troops in the bunker only get low suppression, shouldn't the crew of the bunker (i.e. the bunker itself) also get low suppression in the first place?

A quote about the siegfried line from the book World war II infantry assault tactics by Rottman

Artillery and mortars blanketed the target (Bunker) and adjacent bunkers as the assault troops moved into position; these fires would not damage the bunkers, but suppressed the infantry dug in around them and blasted away concealing vegetation.

Note dug in troops are suppressed but not the bunker itself.

Don't forget shellshock varied greatly between armies. In Armies where it was treated it was very common, In Armies where the treatment was a bullet in the head it was a lot less prevalent. If crewed by poor quality troops the suppression will take a lot longer than 2 moves to wear off.