Is this sentence ambiguous?

A. You were both in the garden, you left, he continued to work.

You would need more context for B to be clear but you could use it that way. "Despite it not being his assigned area i left him working in the garden." Still kinda clunky but makes sense to me.

A In order for it to be B, you would need to change “left” to “let”…. i LET him work in the garden

a and "I made him work in the garden"

The sentence "I left him working in the garden." is somewhat ambiguous, but it more naturally suggests (a)—that you left the location while he continued working there. However, depending on context, someone might also interpret it as (b)—that you allowed or assigned him to work in the garden. But this interpretation is less common unless the context makes it clear (e.g., if you were in a position of authority over him). If you want to remove ambiguity: For (a): "I left while he was still working in the garden." For (b): "I let him work in the garden."

You were both in the garden. You left. He is still there working.

I definitely would not read that as "I allowed him to work in the garden," only that he was working in the garden when you left. There are some cases where "left him" might read as "allowed him," but it's a fairly uncommon usage (for example, "I left him be"). I think it would be unlikely, without further context to clarify, that someone would interpret it that way.

A. There’s no implication there is any sort of permission being granted. Doesn’t even have to be your garden lol

just a

a. like you passed off responsibility of tending to the garden to him.

It's more like "He was working in the garden, and I left him there".