I couldn't tell you which update caused the solved icons to disappear, but they are gone. The screenshot below shows a forum in which all of the topics are marked as solved except for two.
As far as I see they are already back.
Some, but not all. It's really bizarre.
The bottom two items in your screenshot are marked as solved but are not showing the icon. There are others as well.
Ok, this issue is also fixed. The problem maker was the hosting service MySQL server again. For some reason it doesn't bring fresh information from database if the SQL command is based on more than one condition and these conditions are not indexed in table together. We've added this index in your according table of your database and in wpForo core for future releases. Now it should work fine and even faster than before.