Activity pages are aggregating sites even when annotations are anchoring correctly

https://hypothesis.zendesk.com/agent/tickets/7543

A user reported that annotations on https://forums.welltrainedmind.com/topic/694808-what-coursessubjectsskills-are-absolute-musts/ and https://forums.welltrainedmind.com/topic/694616-so-depth-without-rigor-how-to-do-it/ were both appearing under the same site group on her Activity Page. Both sites seem to have document equivalence (same annotation.document_id), possibly due to the canonical tag, but annotations are being anchored correctly. This seems to only be affecting the grouping on the Activity Page.