I think that Ribbons' denying Tieria access to Veda was a function that had to be constantly maintained by Ribbons. Once Setsuna did the 'GN Burst' stuff, it disrupted Ribbons connection through the quantam brainwaves and allowed Tieria to regain access. Though this doesn't explain why Tieria and Regene could speak through quantam brainwaves and access Veda while the other innovades were being distrupted.

Another theory could be that since Regene and Tieria share their conciousness to a certain extent, that they could use each other to access Veda together. Regene was never said to have lost access to Veda, and Tieria was right next to Veda. They could have used each other to complete the missing pieces needed to regain access.</theorycrafting>

tl;dr, creepy talking corpses are creepy.