It’s easy to figure out if the issue is coming from the theme or not: simply activate a different theme and see if the issue persists. If it does, start deactivating plugins one by one until you find the culprit. Start with the plugins that offer memberships or their own avatar replacement functionality which are the ones that usually conflict with One User Avatar. I would leave replacing the get_avatar function as a last resort, as it involves modifying files that are usually overwritten during updates