[RPG] Can damage taken while in polymorph break concentration

No.

It may help to consider this in a step by step approach:

1. The giant ape gets hit for 50 damage while maintaining concentration on a Polymorph spell.
2. The giant ape attempts to make a concentration save to maintain the Polymorph spell. This save is attempted using the giant ape's constitution score, not the Wizard's.
3. The concentration save fails.
4. The polymorph spell ends and the wizard remains.

The Wizard only appears at step 4, and hence cannot be a target for damage before then. The exception to this would is when the damage exceeded the ape's remaining HP, but in that scenario, the wizard would have come out of ape-form anyway, without having to make a constitution saving throw to maintain concentration.

No saving throw is triggered

I found two relevant Q&A twitter posts from Jeremy Crawford (the Lead Designer and Managing Editor for DnD 5e) that shed some light on this. However, Jeremy's tweets are no longer considered official, but they can provide some guidance into his thought process at the time he wrote the tweet.

Question:

if a raging barbarian makes no attack, but takes one damage that is reduced to zero, does rage drop?

Answer:

Taking 0 damage is the same as taking no damage.

If you took no damage, you didn't take any damage.

Taking 0 damage does not count as taking damage, therefore it shouldn't trigger any effect conditioned upon taking damage.

I also found a specific example of a different effect that triggers on damage (in this case, drow poison from the DMG):

Question:

If the Battlemaster maneuver parry, reduce the weapon Drow damage to zero, the poison damage still work ?

Answer:

Drow poison in the DMG is delivered by piercing/slashing damage (0 dmg = 0 poison).

To me, this lays out a clear line of logic. Dealing damage applies Drow poison. If you deal 0 damage, it does not apply the poison according to Jeremy Crawford. Therefore, dealing 0 damage does not count as dealing damage for effects that trigger on dealing/taking damage. This would logically include concentration checks.