Why monitors operated without heavy escorts near minefields

Was it normal to send monitors out without heavy escort because larger ships were too vulnerable near the minefield?

Yes; the mines were the principal threat, and sending heavier ships into the same area could compound the danger rather than protect the monitors.

The minefield itself explains the apparently weak escort. Mines were the primary danger in the operating area. A heavier covering force would have exposed more valuable warships to the same hazard. The loss of a cruiser to a mine in related follow-up operations showed that the risk was real.