One glaring thing i noticed that people have missed is that Yasuke had no last name (family name). In ancient times, Japanese peasants didn't have last names only the nobility and the samurai did. So the fact that Yasuke has no last name/family name and was never given one means he was never accepted as a samurai. (besides the fact that you know....the title of samurai was passed down through blood and you couldn't just become one anyways)
Of course this is just if Yasuke did exist, but he didn't.
I tried to google like someone getting elevated to samurai. I am not a weeb, so I don't know. But I couldn't find any examples. That seems like a glaring issue with the Yasuke samurai story. The first sentence on wikipedia says they're a hereditary military class.
Towards the end of the samurai as a caste, it was all but hereditary. However, that wasn't always true in earlier eras there was more social fluidity. It is possible that someone could perform some exemplary deed and be "elevated" into the samurai caste. Elevated is in quotes as there could considerable expenses to maintain that rank. By Nobunaga's time, the deed would need to be truly exemplary and such a request would need to go through and be approved by the shogunate bureaucracy.