After reviewing this, I'm pretty sure this is more of a quirk than it is a bug.
Some tail types are locked from transformation and they are any tails with a combat implant, nine-tail fox variations, cunt snake tails and cockvine tails. If there is a check for whether or not they can be transformed, they will not change. Additionally, when you initially obtain a cunt tail or cockvine tail, they are initially set as parasitic tails, which are separate from the type and does not relate to the locked status. Some transformatives or events will remove the parasitic quality and leave the tail type unchanged (even if it changes the tail's genital type, like Siren's Bounty as mentioned).
In the case of Foxfire, it only changes any unlocked tail types or parasitic tails and converts them to a vulpine tail type, having genitals or otherwise, and then clearing the parasitic trait. Since your character's tail lost the parasite trait prior to using Foxfire, you were stuck having a tail type that could not be transformed. When a V-Ko (or any other character/event) removes a parasitic tail, the entire tail goes with it.
The reasoning behind the locked tail type being the way it is is that "pure" cunt snake and cockvine tails can't be removed (it has been this way since the introduction of the cunt snake), so it has lore implications. When Foxfire was added (or possibly another item) where a genital tail can be made non-parasitic (mainly to please players who find parasites unappealing), the "parasitic" trait, and eventually a tail flag for it, was added. This compounded the complexity of defining what is and isn't a cunt snake or cockvine as by their nature they should be parasitic. Since the player can now change the shape of their genital tail, the tail type locks became more of a barrier to work around with the order of item usage.
The solution I've implemented for the next release will be to add the parasitic flag in addition to the tail type checks. So if the character has a cunt snake or cockvine tail that is also parasitic, it should be considered as locked. Non-parasitic cunt snake and cockvine tails should be considered unlocked. Hopefully this makes tail changes a little more broadly accessible while keeping to the lore.