When $lookup writes to a dotted as path such as "a.b", it can replace the parent field "a".
For example, if the input document is:

{a: [{d: 5}]}

then $lookup with as: "a.b" coerces "a" into:

{b: []}

This removes "a.d". The optimizer currently treats $lookup as only modifying the exact path "a.b". Because of that, it may incorrectly push a later $match on sibling path "a.d" before the $lookup.
Example pipeline:

[
  {$lookup: {from: "joined", as: "a.b", pipeline: []}},
  {$addFields: {"a.c": "$x"}},
  {$match: {"a.d": 5}}
]

Correct behavior: $match runs after $lookup, "a.d" no longer exists, and the query returns 0 documents.
Buggy behavior: $match is pushed before $lookup, sees the original document with "a.d", and incorrectly returns 1 document.

I reproduced on 8.0, 8.3 and master, but it probably goes back much further.