`--dry-run` flag in `occ db:add-missing-indices` partly ignored

occ db:add-missing-indices --dry-run

did not work, at least not for this operation:

~# occ db:add-missing-indices --dry-run
Check indices of the share table.
Check indices of the filecache table.
Check indices of the twofactor_providers table.
Check indices of the login_flow_v2 table.
Check indices of the whats_new table.
Check indices of the cards table.
Check indices of the cards_properties table.
Check indices of the calendarobjects_props table.
Check indices of the schedulingobjects table.
Check indices of the oc_properties table.
Check indices of the oc_jobs table.
Check indices of the oc_direct_edit table.
Check indices of the oc_preferences table.
Check indices of the oc_mounts table.
Check indices of the oc_systemtag_object_mapping table.
Adding systag_by_tagid index to the oc_systemtag_object_mapping table, this can take some time...
oc_systemtag_object_mapping table updated successfully.

Hope this helps

Originally posted by @ernolf in https://github.com/nextcloud/server/issues/39379#issuecomment-1634992671

Start at the occ db:add-missing-indices command implementation and reproduce the shown --dry-run invocation. Trace the index update path and verify that dry-run mode leaves the database unchanged while still reporting the checks.