Actually, Yui called him Takaomi-kun on chapters 131 and 132 as well -- though I think the scanlated version put in "Saeki-sensei" which might be the reason for the confusion:-
Chapter 131
"Is it true that Takaomi-kun disappeared!?"
Chapter 132
"Takaomi-kun might be worried..."
And because I got really curious as to when Yui started to call him by his first name, I went ahead and dug up the earlier chapters.
Yes, anon-chan man went all the way down the rabbit hole 😂
It seems that Yui was still referring to him as "Saeki-sensei" until Chapter 35
But on Chapter 43 ( Banchou & Hayasaka's misunderstanding LMAO), Yui called him "Takaomi-kun" after
eavesdropping learning that Mafuyu & Takaomi are childhood friends.