DE comes into pool after pump turns off

[Susan]

My pool was just resurfaced with fiberglass.

Prior to this, we backwashed the DE filter then drained the pool through the waste function on the multiport valve.

After refilling the pool, we restarted the pump & reloaded DE as usual. But when we turned the pump off, we noticed a cloud of DE coming up from the main drain and later, through the skimmers.

What's the most likely cause of this? Is it more likely that the DE grids were damaged during startup or is there potentially a plugged line (creating vacuum and pulling DE off), or could it be something else?

We did have the gaskets in the multiport valve replaced, but this did not solve the problem.