The way the lids are designed, there is a small void around the edge of the lid between the glass and the metal rim.
Moisture accumulates in this void during cooking and washing and does not drain or dry for several days, causing unsanitary food/soap/bacteria/mildew build up. Many foods deposit a residue inside this gap when they are cooked. For example, cooking something in the pan with a sauce, as it bubbles up / steams this gets into that gap at the edge of the lid. There is no way to clean this gap because the glass/metal are too snug to fit any cleaning utensil. Washing by hand or in the dishwasher doesn't work. Even if, hypothetically, there is a cleaning chemical that can dissolve everything inside the void, it wouldn't be able to drain out fast enough to take all the gunk with it (literally takes 2-3 days to drain/dry).
Customer service has confirmed that the lids are designed so that the gap is not fully sealed. They have suggested several methods of cleaning but none of them work. I have been refused a refund and told this is not a warranty issue. However, I am submitting for a warranty assessment.