Some creatures, like Black Puddings, have the Amorphous trait, meaning that they can move through very small spaces. But these creatures don't all have the restrained condition immunity.
What happens when you successfully throw a net at such an amorphous creature, provided that its not immune to the restrained condition? Will they be able to just move away from the net without any penality due to being amorphous, or will they suffer the Restrained condition as normally given by a net?
This assumes that the creature doesn't passively dissolve the net when it touches its skin, or some similar destructive effect.
The Amorphous Enemy is Not Affected
We know the following about nets from the item's description in the Player's Handbook (bold added):
Although "formless" isn't a tag or formal term (like "shapeshifter") which clearly applies to certain creatures and not others, we have a hint by how rarely it is used. The only time the word "formless" is used in the Monster Manual is in the following passage (bold added):
As such, we have evidence that oozes and slimes are intended to be considered "formless". And as such, nets will not affect them at all.