I have been researching and developing many glazes over years for food safety. Talking about terra cotta products, as the piece is low-fired, the item/glaze can sometimes
still react to temperature changes.
This reaction may cause your cups crazing. These hairline cracks are completely normal and can resemble tiny crackle patterns. However many hobbyists use them as a tableware product. I strongly believe crackle glazes are artistic glazes, not hygienic, and make the ceramic piece vulnerable. This is the reason I advise washing terra cotta products on hand and allow them to adjust to temperature changes.