Think I may have isolated the problem.

I spread 300 lbs of bentonite chips around mortar cap surrounding the drain pipe but the leak didn’t stop. Today I decided to open the drain valve and lower the water a bit to see the pipe and mortar better. I discovered that when the ice pressured the pipe, it actually opened a significant gap between the mortar cap and the PVC.

Draining the pond another half a foot and will seal the gap between the pipe and the mortar shield tomorrow. I’m pretty sure that is the source of the problem and am hoping this will solve the issue.