I've recently experienced this problem with the lid of my Tetsubin. It is a Kunzan tetsubin unglazed inside. I’ve used the it once-twice a week since I bough 3 years ago. I’ve always used the Tetsubin kindly following the instructions.
The bottom of the lid glaze starts deteriorating in this way in the last month. The vendor said that it is normal because this glaze is a protection of the lid made with urushi (that is not toxic) and in frequent use may degrade. I cannot contact directly the procuder because he has not a website, but my concern is that I'm not sure it is urushi, so it can be harmful. As you can see in my finger the enamel breaks in small pieces and it is obvious that while boiling it can fall on water.
Actually, as you can see, this glaze appear very different from enamel or glazing of cast iron teapots, but it seem strange that urushi can detach due to its strong adherence (e.g. in kintsugi). I've found on Suzuki website a maintainance advise that suggest to treat the outside of the kettle in order to maintain the external urushi treatment, and also in this case he substain that urushi can detach.
"3. Usually please wipe the outside of your iron teakettle with a dry dishcloth. And it is also recommended that while your iron teakettle is hot, you sometimes wipe the outside of your iron teakettle with a dishcloth dipped in green tea. Then, peculiar gloss will be produced on the surface of your iron teakettle. This is because Urushi, Japanese lacquer, is printed on the surface of the iron teakettle. Urushi is an outstanding coloring material which has an anticorrosive effect, but it will separate gradually as time passes. Nevertheless, if you continue to use your iron teakettle, it will begin to show the tasteful color before you know it. This can also be called to be the magic of Nanbu-Tetsubin."
Someone of you experienced something similar? The lid of my tetsubin is in cast iron. It is your lid glazed?
I want to specify that the vendor is very serious and professional, it ask and confirm me that also the manufacturer confirm is urushi, but I would like to know if you have similar experience on this type of tetsubin and how to avoid this issue..