So back during the last part of the Archon Quest Yae tells us that she's also a subscriber to Ei's ideal of Eternity. The only difference is that she doesn't believe in a static eternity.
Let's see if that plays out.
In her voicelines she (well she throws tons of shade at Ei 
) talks about eternity like she's good with it but then goes into how it's impossible. Change is how the world works. She also states that eternity is boring because you lose out on all the possibilities when you force everything to stay in the one possibility you found it in.
Everything about her speaks to being the God of Impermanence. (Or I guess Transience is the word MiHoYo uses.) She even likes sowing a little chaos to keep things interesting. She also makes it pretty clear that she's actually Ei's old mentor. In the Archon Quest itself she's the one that taught the Archon how to place herself into Makoto's sword. She also constantly calls her immature and pig-headed likely stemming from her failure to impart the ideal of Impermanence into Ei. (I feel like the new quest shows she ended up having more success with Makoto but more on that some other time.)
And this idea of being Ei's mentor not her servant (I mean I guess after the overthrow she did become the servant) carries over into another theory I had. Yae's line about Euthymia could be interpreted a different way. We already saw that the Shogun who was programmed to detect if Ei was suffering from erosion accepted Ei's new ideals. But there are times when Ei does actually seem weird. She claims she has nothing to do with the Visions but we learned previously from Zhongli (an original Archon) that they do. Ei also doesn't realize desserts aren't the whole meal and thinks that you can just replace your teeth if you get cavities. (which is really funny for a person fearing erosion because tooth decay is when your teeth are eroded by stuff like sweets 
)
I have a feeling hiding herself away in Euthymia caused some memory loss or affected her ability to think about the events leading up to her going into it. It might have also had the added effect of making her regress a little so she behaves more impulsively despite being at least a few thousand years old.
Tips and Tricks from a Super Casual Genshin f2p
