the extended char set storage is controlled within your mysql database. it’s also passed there through php. and stored in the defined charset collation format. all of those things are controlled on the server side and not any setting which is within the UBB.threads software except for the method at which the chars are checked before being passed from the webform to the database storage. That is handled by a php module and the php function which parses it (code within the ubbthreads software). this has been updated over the years. but this would be the only piece of your current forum software which definitely has not been touched/changed (according to you) at all. so what has probably changed is a server side php module update or mysql charset storage adjustment.
maybe the issue may even be the browser converting the char from the outdated iso-8859-1 set of tour forums to the utf-8 set. though I dont believe that is related at all, since it still displays in the preview.
tldr; something on your server was updated. maybe it might be time to use human searchable fractions rather than extended chars, which are primarily only available to word processors. mobile device and traditional 101/104 keyboard layouts do not have dedicated fraction keys. In addition, fractions are rarely, if at all, indexed by the major search engines, and even less likely to be used when a human is searching for specific results.
these are the only answers I can come up with. I wish I could be of more help, but that’s all I have for you, which is based on my experience and the information already shared in the above posts.
(excuse my caps/typing. posted with my smartphone)