Area to the right of the 'main' swamp zone, behind some rubbery strips... but it's a 1/2 chance of the area spawning so it might take a visit or two...!
The one with lots of goo pools and bugs?
Also, during boss fight. If you loose and end up with full corruption after 'leaving' off the plinth, you don't get the 'bad end' and instead get teleported back to camp as normal.
So: 1. is that intended? 2. Since you now have the new fancy suit, is the helmet meant to not be visible?
No, the other one. It has a dock with a serpent and a locked rubber chest.
Please make sure you update to v0.109, and this error may hopefully have been fixed. (you will need to repeat action, it cannot fix on a save where it's already happened, only prevent it in future)
(and yes, helmet is not meant to be kept)