I could've sworn it was the other way around with reactions being resolved first unless it was phrased in a way that makes sense otherwise (like taking damage and casting Hellish Rebuke), as otherwise how could someone attack a person who moves out of reach if they are now some undefined 10-movement speed amount of space away without a reach weapon, but perhaps that might just be the exception then?
We've always house ruled that any reaction attack typically happens first. Chalk it up as one more houserule/remnant of older systems at my table if it's the case. shrugs