פירוש על עבודה זרה 150:15

Glassware can be repaired, and was indeed repaired, in a world in which glassware was very expensive, and is thus like metal. Just as metal utensils must be immersed, so must glassware.

Glazed utensils before they are glazed are basically just earthenware and earthenware vessels need not be immersed. But by the end, they are like glass and glass vessels must be immersed. So there is a dispute as to whether they need immersion like glass utensils, or not like earthenware. The rule is that they do.