Shortly after installation of the solved plugin we noticed some topics having the autoclose message " This topic will close 7 days after the last reply." at the bottom, but there was no posting marked as the solution.
Are you able to reproduce this? The solved plugin will only add the topic timer if the “solved” button was clicked and SiteSetting.solved_topics_auto_close_hours is enabled. I think something else it at play here for your instance. Perhaps you have auto close configured for the category?
I’m thinking of storing the topic timer id in the topic’s custom fields instead and avoid having to add an additional column that is only used by the solved plugin when SiteSetting.solved_topics_auto_close_hours is enabled.