Explorer Face Bug Fix
watchOS updates routinely contain bug fixes and performance improvements. In watchOS 5.2.1, Apple has patched a bug that could cause problems with the Explorer watch face for some users. (Presumably, watchOS 5.2.1 also contains smaller bug fixes not mentioned in the release notes.)
Essentially, the software bug — which was introduced in watchOS 5.2 — caused the minute and hour numbers on the Explorer face to disappear. In the latest watchOS update, that bug has now been fixed and the face should work normally.