I've always had troubles with tasks with the calendar which is what the default calendar and tasks sync with as I understand it. I don't know exactly long it's been beta but it's been years. It's one of my big pet peeves about Google. You would think a company this big could manage to finish a project before moving on to the next and the next and the next leaving this one in the dirt. I now use a separate task app. There was a GTasks app in the Play Store, you might like it. I don't remember if it or how it interfaces with the default calendar, been too long since I've tried it.
Currently I use DigiCal and Life Reminders but I haven't found the perfect app for either the calendar or tasks yet. Life Reminders works really well for me, I just wish it were more attractive and adult looking. DigiCal does tasks bur not enough options for me.
Notes from Deb's 2014 Note 10.1