It looks like a unionWith query should have reached the memory limit and failed when $group is pushed down, but it suceeded. This is most likely due to the difference in reporting the memory limit in HashAgg as it differs from the classic engine.

https://logkeeper.mongodb.org/lobster/build/f1bc993303085d81713acaa781df07b1/test/6195b3b39041307c2347fe84#bookmarks=0%2C34%2C180&l=1

The investigator should verify this and determine if the test can be changed or we need to revisit memory usage checks in hashAgg.