This will be fixed in a maintenance patch later tonight, but will need to await for approval from IPS.
FWIW, this method is documented as only accepting arrays, so it should never be passed a string, though I see IPS manually casts $set to an array if it's not one in the internal method.
/**
* Set tags
*
* @param array $set The tags (if one has the key "prefix", it will be set as the prefix)
* @param \IPS\Member::NULL $member The member saving the tags, or NULL for currently logged in member
* @return void
*/
I'd prefer @bfarber or someone kept that bug report open and rather than closing it as a third-party issue address the fact that the RSS class isn't recording tags as it should, especially given that what was passed to the method was seemingly just '[]' (an empty array incorrectly represented as a string), but nonetheless, I'll address it. I'm thinking there are still underlying issues in this class that need fixed, though. This is why I'd rather IPS move towards strict type checking.
I can certainly look into implementing this in the next feature release! Keep in mind that feature updates going forward are likely just going to be for IPS 4.5 however.