on author and publish instances TagManager successfully resolve tag but it fails to resolve tags when we access through webserver. We are resolving tags using fully qualified tag id(ex: mynamespace:car/color/red).
TagManager tagManager = resolver.adaptTo(TagManager.class);
Tag tag = tagManager.resolve("<fully qualified tag id>");
Here resolve(-) returns null value.
Do you have any sling mappings for web server URL? If yes, try removing the mappings and try again.
Also, setting up a DEBUG logger on the following classes will provide more info on what's going on:
Please review this post Unable to resolve tags in AEM 6.3 using Tag Manager