Most probably it happens because of the Align to Pixel Grid option switched on, as per comments. Try to disable it and see if it changes anything.
Can you try without this place selected?
You have Align to Pixel Grid switched on. See the attached screenshot of your video.
Click that off and try again, you may have better luck.
A word of caution though, sometimes Illustrator's 'smart' guides and snapping don't work as well as we'd like so you may still struggle a little bit.