Why is Present Perfect is used in the first part of the following sentence:
'In past downturns I've done well, but this downturn has me stumped.'
Is it because 'in past downturns' can be changed for something like 'so far'?
That's right. The person is talking about an ongoing situation. Even though it's not continuous, he's talking about his performance during downturns.
The reason for the question is that my students might say that the word 'past' is a typical marker for Past Simple, and here we use Present Perfect.
They might. You'd need to tell them not to think so literally.
(Actually, I'd question whether the word 'past' in a sentence is an indication that the sentence should be in the past simple. I'm sure you're not teaching them that.)
If your students say, "The word "past" is an indication that the past tense should be used", you can point out the contradiction inherent in that statement.
"Your past will be taken into account."
Thank you in advance.