Very interesting topic that triggers both my gamer and dev sides (not an expert dev by any means tho).
My friends know that when I'm facing a particularly bull*hit challenge in a hard game, or even ones that are not even supposed to be hard, I'd usually (very maturely and pretentiously) go "Plaese playtest your game before selling it to me." or "I wanna see the game designer do this in no damage!".
Most of the times this is obviously a a skill issue from my part lol but sometimes we all got vindicated by the dreaded "balance patches" that somehow mean that you didn't beat the game and stuff like that. A skilled driver can and *will* be able to drive a clunky old broken car home safely, but that doesn't change the fact that car *is* broken and needs to be fixed (Promised Consort Radahn has entered the chat).
So basically yes, obvious bug-checking aside, when it comes to difficulty and all that comes with it I absolutely playtest the s*it out of my own game and some months ago I had friends playtesting a platformer I published last week. As a side note: watching your friends playtest your game can be hilarious and a fantastic learning experience, especially if they're different kind of gamers, like some are more used to old-school platformers (like mine), while others were surprisingly struggling with basic stuff, but that's another story. Back to me, while my game was never supposed to be particularly difficult, it does have some more challenging gauntlets and I made sure I could beat those (like everything else) without taking any damage, over and over, sometimes doing mistakes on purpose to see if there's even room for small mistakes (which there should be in my opinion).
The "problem" with playtesting your own game and beating it is that... well, you made the game, you basically mastered it before it even started existing, you know everything that's coming for you, you know the hitboxes, what's going on behind the scenes, the pixel perfect tricks (that a player *can* learn but not as fast as you because YOU made it). Obviously that's not true for every genre I'm sure (I'm limiting myself to platformers because that's the kind of game I made), like OK I developed a roguelike but randomness still gonna be random, so to speak, in that case the playtesting part goes beyond "act and react" and reaches different layers of depths (which is why I don't make roguelikes: I'm dumb -I do play them tho).
Short answer: YES, despite the unfairness of me knowing everything the game is gonna throw at me (because I made it), I absolutely playtest my own games, even doing no damage runs, because I want to make sure everything is fair.