No index support for filtering on array fields inside $expr operator

I have a collection containing the following documents:

The index below is defined:

I'm testing an aggregation with a single $match stage:

It returns the second document as expected, but without index support:

I discovered this strange behavior when testing a $lookup stage with an internal pipeline, where $expr operator is a must.

BTW - I'm aware of this issue. I have tested non-array fields and indeed indexes are used for them.