The proper action with such people is to send a follow up mail. "I summarized our decisions from the coffee break...".

Sure, instead he'd just change his mind three times distributed across hundreds of Slack messages, and you'd still have no tracked resolution.

When you have a decision born of a conversation, the only useful thing to do is to write that distilled, focused result on some median other than the discussion itself.

This problem ultimately has little to do with remote vs colocated; The same applies whether the discussion happened in verbal discussion, a meeting, an email chain, messaging, etc.

That problem is easily solved, look at the most recent message. Though a lot of the problem was caused by incredibly incompetent management.

Tip: after a meeting / conversation, create the user story as you understand it based on the conversation then tagged your manager on the item and ask for confirmation. Ideally, you'd have a product manager who keeps an easy to read backlog; but if you don't then you need to take matters into your own hands.

If you don't have someone taking detailed meeting minutes, the meeting might as well not have happened. Always take notes and send to all participants promptly to verify accuracy. Isn't this Running A Meeting 101?