I've seen a lot of creators talk about this. From what I've noticed, Shorts don't necessarily hurt long-form videos, but they often attract a different type of viewer. Someone who watches a 20-second Short isn't always interested in a 10-minute video.
That said, Shorts can still be great for growth if they're related to your main content and give people a reason to check out your longer videos. The biggest mistake is treating them as two completely different channels.