Armor stands with the "Marker" tag set to 1 can be moved by water, pistons, and explosions

Incomplete without a description for steps to reproduce.

Um, you place an armor stand, set the "Marker" tag to 1, and pour water on it. Is it really that hard?

The marker tag is only meant to make it invisible and have a small hitbox. I'm not sure what you think its supposed to do.

FYI, this is WAI. The armor stand is / always is an entity.

Well, I think it's pretty clear that armor stands with the "Marker" tag are supposed to replace wither skulls as markers in custom maps. As such, they should be immovable by non-command causes, just like wither skulls are now.

Duplicate of MC-75197
Please do not duplicate your own issues!

Um, this is MC-75197.

Oops! My mistake! I meant MC-75195

Just because they both refer to armor stands doesn't mean they are duplicates. They are separate issues.

Use the NoGravity tag if you don't want the armor stand to be affected by physics. That's what it's there for.