I know it's a bit weird, but the glossary style must be added to both the section and the description list. In other words, you need:
.glossary.adoc
[source,asciidoc]
----
[[glossary]]
[glossary]
= My glossary
[glossary]
word1::
My definition here
----
You could also type it using shorthand for the id:
.glossary.adoc
[source,asciidoc]
----
[glossary#glossary]
= My glossary
[glossary]
word1::
My definition here
----
It probably would be nice if Asciidoctor caught a glossary list that is outside a glossary section. Perhaps file an issue to emit a warning? Otherwise, I think we get broken DocBook.
-Dan