I am thinking on the lines of two possibilities:
Option 1: The Sleepless. I think this one most likely. They seemed to want to protect what they have in Aimia (including apparently a large cache of soulcasters) - perhaps to prevent them being used. The Sleepless always seemed like spren themselves with the way they are sort of just possessing a bunch of cremlings. Theory: Could it be that the soulcasters spren Navani observed are "the Sleeping ones" vs the Aimians are "the Sleepless" - they are the same just not captured and therefore still conscious, hence the Aimians hoarding soulcasters to protect their kin. Similarly, could the Aimians be aware of what happened with Ashyn and are trying to warn Navani not to create these spiritual realm fabrials as that would have similar disastrous effects?
Option 2: Cultivation herself. We know in Mistborn
so I think Cultivation should be able to somehow manipulate a gem directly to write or to alter text from spanreed. Some spren call her Mother, she may really view them as her children and therefore be against Fabrials. Additionally she may not trust Honorspren now that Honor himself is not around and think they are misguided. She doesn't generally act this openly but does in exceptional world-changing cases (e.g. Dalinar) and maybe she saw something with her future-sight that indicated some pruning may be needed here. Again she would be aware of what happened on Ashyn and could be trying to avoid the same happening on Roshar.