What thread does Combine `.collect` method emit on?

What is the expected thread on which a .collect operation in Swift's Combine will emit?

Specifically I am seeing this code crash in the second precondition, but not the first:

return Publishers.MergeMany(publishers)
    .handleEvents(receiveOutput: { _ in
        precondition(Thread.isMainThread) // <- This is fine
    })
    .collect()
    .handleEvents(receiveOutput: { _ in
        precondition(Thread.isMainThread) // <- Crash is here
    })

It is as if the .collect operation is picking a different thread to use, even though the final publisher in the MergeMany must have emitted on the main thread. I have deduced this behavior via crash logs uploaded from Firebase. Can anyone explain this observed behavior?