I found a little hint.
Property ::Page Tags
Description: Provides a way for a page to associate to another page i.e. backward compatible tagging.
So “Page Tags” are related to the Logseq-MD to Logseq-DB transition.
Using my Gemini AI ‘Gem’, it suggests
So when round-tripping a Graph between MD and DB, “Tags” in MD convert to “Page Tags” in DB?
But “Tags’“ in DB are “New Tags” with structure and inheritance, so are not exported back to MD.
If you want reverse-compatibility, use “Page Tags” as well in DB, so they are exported as “Tags” in MD.
Untested theory: may be AI hallucination?