You need to move your return lines. In your picture they appear to be a couple inches below the teeth on your overflow box. Return lines, especially on larger tanks need to be near the water surface or at least about the same level as the overflow teeth. When pumps are off they will back siphon and with them that far below the overflow teeth in a 220 you could easily overflow the sump to the tune of 20 gallons or so. You mentioned that the lines were out of the water but that’s only because they had already drained a bunch of water. The water level in that picture is right at the return lines.
If your closed loop is leaking then that is a separate issue but to me it looks like your main culprit is the return lines being too low.
If your closed loop is leaking then that is a separate issue but to me it looks like your main culprit is the return lines being too low.