That isn't my understanding of a leaky abstraction. An abstraction leaks when it's supposed to always behave in some certain, predictable way, but in practice, sometimes it doesn't. When does an async function not behave the way it's supposed to?
My understanding of a leaky abstraction is that the abstraction itself leaks out details of its design or underlying implementation and requires you to understand them. What you seem to describe is a bug, edge case, or maybe undefined behaviour?
For example, an ORM is a leaky abstraction over an RDBMS and SQL because you inevitably have to know details about your RDBMS and specific SQL dialect to work around shortcomings in the ORM, and also to understand how a query might perform (e.g will it be a join or an N+1?).
I don't really think that the async or blockingness is the leak, but that the time taken to process is not defined in either case, and you can leak failure criteria either way by not holding to that same time.
People can build to your async process finishing in 10ms, but if suddenly it takes 1s, it fails
