Why doesn't Asaj Ventress have a Sith tag?

Grievous was trained by Dooku the same way asajj was: as a fighting force for the separatists. The rule of two is a constant:

Dark Lord of the Sith: Palpatine - Darth Sidious
Sith Lord Apprentice: Dooku - Darth Tyrannus (at the time)
Sith Lord’s Disciples: Ventress, Grevious

Later, Anakin replaces Dooku as Darth Vader, just as Dooku replaced Darth Maul. Vader took on disciples that were not recognized as Sith as well, like Galen Marek (Starkiller). This isn’t canon anymore, but it falls under the same general concepts / structure.

*marek was a secret apprentice of Vader as a Sith, publicly recognized as an agent / disciple. When Emperor Palpatine confronted Vader about it, he faked Marek’s death and sent him on missions in secret to plot against the Emperor, planning to take over. Dooku never had plans like that with Ventress, but Darth Maul did!

Cannonically from Star Wars Clone Wars tv show, Maul resurfaced after being in Exile from being defeated by Obi-Wan in Ep 1, taking on Savage as a formal apprentice. When confronting Darth Sidious, he was asked if Savage was his apprentice - when confirmed, Sidious then referred to Maul as a “rival,” practicing his own structure for the Rule of Two, since he’d been through formal Sith Training as Sidious’s apprentice. When defeated, and was sent back into exile until resurfacing in Star Wars Rebels TV show. At this point he was no longer identified as a Sith: just a dark side Force wielder.