I've experienced this for years. I've even recorded my screen to show the problem. I have lost money, as you can imagine.
Recently, the problem is worse; you can set a stop at open, check your position and it's set, and then several minutes later, go back and it's gone..!
I have never had a response from IG.
I used to build trading systems very similar to IG's and I'd consider this a CRITICAL bug in the app. I have used different browsers and more recently I use a Macbook, so it's a bug in their JavaScript.
I should really leave IG, it's way, way too dangerous to keep trading, but I just have not got around to it.