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Is this a problem only on that forum or on all Discourse forums? Were you, for example, able to reproduce this on try.discourse.org? If it happens only on that forum, it might be a problem in the configuration of that forum which cannot be fixed by the Discourse team; you’d need to report this to the forum’s admin instead.
You can also try first to see if it still happens in safe mode. Safe mode disables customizations, so if it helps, we know it’s a customization the admin will need to take care of.