serde flatten causes parent element propagation

[outergod]

Related Gist: https://gist.github.com/e-user/43b4dedce72c4fd80757fb262a6a2f36
Expected outcome: Code from Gist runs without errors
Actual outcome:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `"<root><another><root/></another></root>"`,
 right: `"<root><another/></root>"`', src/main.rs:42:5

This is the most condensed version I could figure out to reproduce this error and included a JSON version to demonstrate the bug is most likely not in upstream serde itself. It works at arbitraty depths, not just the root element - one a flattened HashMap gets involved, its parent element somehow gets propageted to its own as well as all siblings' child elements as well as its descendents.

Addendum: It doesn't have to be a HashMap which is flattened, it works on any non-unit struct:

pub struct Props {}

[Mingun]

Duplicate of #343