@Mark Takata (Adobe) (and anyone else interested) here's my assessment of that

runAsync

bug I found y/day: https://blog.adamcameron.me/2023/02/coldfusion-bug-in-runasync.html

So the ACF bug doesn't occur if you use function notation to record activity rather than a closure?

@Adam Cameron wanted to let you know, you've definitely got the engineers buzzing over this bug. They actually found an even simpler repro that does what you showed and they are digging into the code to figure out what in blazes is going on in there. So cheers mate for the fantastic find!

Glad to be of service. Would be keen to see the simpler repro