Upgrade to 15.6.0 fails: relation "index_issues_on_title_trigram" does not exist

Summary

The upgrade from 15.5.4 to 15.6.0 fails:

PG::UndefinedTable: ERROR:  relation "index_issues_on_title_trigram" does not exist

Steps to reproduce

Debian Stable. Installed Omnibus 14.0.12 (CE). Imported source-installation of 14.0.12 via backup. Upgraded to 15.5.4 following the upgrade path. Tried to upgrade to 15.6.0.

What is the current bug behavior?

The upgrade fails, where PG::UndefinedTable: ERROR: relation "index_issues_on_title_trigram" does not exist seems to be the root cause.

What is the expected correct behavior?

Upgrade passes.

Relevant logs and/or screenshots

Attached.cinc-stacktrace.out