Hi luckymethod,
I'm going to address your questions in two ways.
1. Yes, I believe this is a bug and I've forwarded your video and a file on to our sketch development team. I'm able to easily replicate this.
2. And, as for much of the advice in this thread, There are better, more stable methods to build this type of geometry in Fusion. While the lessons author shows a method that I understand you're attempting to follow, his method is not one I would promote as a best or even good practice for parametric modeling sketches.
Here's some different approaches to consider:
Starting at the bottom of the attached image, is the tutorial authors method, which I would not advocate for. Advice I like to give people is:
- always keep your sketches as simple as possible
- distill your sketches down to their most basic geometric description whenever possible, i.e. if you don't need to trim a circle, don't
- the simpler the sketch, the easier time you'll have troubleshooting problems with models and assemblies when the arise.
- If possible leave details, such as fillets and chamfers (as @TrippyLighting suggests), out of the sketch, unless absolutely necessary for them to be there. Those details are far more robust as model geometry than sketch geometry.
- Always remember there's more than one way to achieve what you're working on.

hope this helps,
Jamie Gilchrist
Senior Principal Experience Designer