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Glaze compression is fracturing these pieces. The maker of these had been doing it for some time with success but a change in the clay body recipe started to cause problems. These are his experiments to formulate a body/glaze combination that could survive in spite of the glaze being almost as thick as the clay in some sections.
This mug is made from 325 mesh MNP, the strongest porcelain I have. Since the walls are of even thickness with no abrupt corners or contour changes and the glaze is thinly and evenly applied I thought I could follow a social-media-driven trend and glaze only on the inside. But I got glaze compression time-bombs waiting for hot coffee triggering! Three other mugs failed this same way! But four with this same glaze inside and out were fine. Why? The outside glaze counters the inside one pushing outward. And it closes crack initiation points.
I got lots of pushback on social media saying glaze compression problems are overblown. But I also got stories and pictures much worse than this (especially with thick and drippy glazes). But, some still feel that outside-only glazing can work by carefully tuning the thermal expansion fit between body and glaze. Or even by accident. Either way, there is still an elephant in the room: Glaze fit has to be just right - too much and pieces break, too little and the glaze crazes. That is a problem because it brings intolerance of even slight changes in body, glaze or firing.
Of course, if you make thick-walled ware and the glaze thickness is not really crazy, then you can probably get away with doing everything that I have advised against above.
Buy me a coffee and we can talk