blob brush tool buggy

Thanks for the video. A couple more things to check:

In Settings, Input, do you have palm rejection turned on? It should be on. Also check if there is an issue with screen protector and contact with the Pencil etc. If it's not either of those. Double check your system requirements here:

https://helpx.adobe.com/illustrator/system-requirements-ios.html

If none of that is the issue, report the bug using Illustrator User Voice - go to App Settings > Support > Help > Report a bug, and that takes you directly to User Voice.