When Itachi poked Sasuke on the forehead Sasuke saw it as a sign of affection!?
Yup, because despite Itachi always having used it as a means to gently escape spending time with him, Sasuke always knew that the poke was a positive and affectionate gesture; it was a way for Itachi to convey to his little brother that he still cared for him despite his busy schedule, and that he was going not because he wanted to, but because he needed to. When Itachi and Fugaku were going through a rough patch with each other, Itachi grew more stoic and visibly more distant, and Sasuke was disturbed by this.
During this time, Itachi would again politely distance himself from Sasuke in order to fulfil his duties as a ninja. However, he also poked Sasuke’s forehead while doing so, which was something Itachi hadn’t done in a long time; since before his deteriorating relationship with his father:
And while Sasuke was no doubt frustrated that Itachi’s responsibilities as a ninja consistently got in the way of their time together:
It was still enough to make him feel much better, because Sasuke felt as though the old, affectionate Itachi was back:
Why? Because Itachi poked his forehead again. This is explicit proof that Sasuke viewed the poke as a sign of affection, and that’s how he used it on Sakura and Sarada. The display of affection was always the main point of the poke, and Sasuke used it on them, much like how Itachi used it on him - to politely distance himself due to the necessity of his redemption journey, and his mission to find Kaguya’s traces, but also to demonstrate to them that they still meant the world to him.