Repro.swift

import Dispatch

class Runner {
    var observers = [String: () -> Void]()
    let queue = DispatchQueue(label: "foo-queue")

    func run() {
        let observers = self.observers
        for _ in 0..<20000 {
            let c: () -> Void = { [weak self] in
                self?.queue.async { [weak self] in
                    self?.observers.removeValue(forKey: "")
                }
            }
            self.observers[""] = c
            c()
            for (_, observer) in self.observers {
                DispatchQueue.main.async {
                    observer()
                }
            }
        }
    }
}

Runner().run()