How do hydraulic flat tappet lifters fail?

Doubt the anti-freeze is related to the cam. What causes lobes to go flat is pretty easy to say. The cause of the cause will be determined by you.
Common Causes After Proper Break-in:

• Lifter doesn't spin (rotate)
• Collapsed lifter that won't pump up
• Not enough zinc in the oil with high pressure springs

1. Lifters shouldn't be tight in the lifter bores. When you pull them up an inch, they should slide right back down to the cam.
2. Lifters need to rotate. Sometimes you can take a paint marker and put a line in your push rods up by the rockers. You can verify them rotating with the valve cover off.
3. You need zinc around 1200 - 1700 ppm in your oil. VR1, ZDDP additive, and a thousand other ways to get what's needed