Sparse index utilisation without $exists

https://docs.aws.amazon.com/documentdb/latest/developerguide/functional-differences.html#functional-differences.sparse-index is causing me problems migrating an existing MongoDB code-base that makes use of sparse indexes - some of the queries are now unindexed and therefore take minutes/hours rather than milliseconds.

Is there a plan to fix this compatibility bug any time soon, or do I need to track down every query that relies on a sparse index and fix it?

Thanks,
Tom