I don't think it's luck, rather I think seme has too much street-smarts to already build immunity against poison at this point in his career, he probably has seen all methods of assassination attempts too whether it's directed to him or to those he's working for. More than likely, he knew that uke is already planning something. From the very beginning of their affair, he already acknowledged that uke will betray him because of what he did to his family, he was just playing the game with uke but even he is starting to slip now, both of them are (seme was about to be sentimental and do some backstory reveal to uke in some prev chapters but shook the impulse away, while uke is now getting a bit comfortable in what they have and is second guessing his ultimate plan)
I'm curious too how he will punish uke this time, I think he won't send him away though, but he'd probably do that to the servant boy since that was originally the punishment from the previous incident after uke took the arrow to his knee. He could lock up the uke or something, or just sending the servant boy alone could be punishment enough-- and since it's such a light punishment from an outsider's perspective considering the weight of the offense, this could spread rumors on the power that uke has over seme, which might attract other powerful, underground figures who want to kill him too or overthrow the government to use uke as their weapon, a replacement for those 2 political dudes who offed each other by the river. Also, I just thought that if seme did lock up uke as punishment, 2nd seme might think of it as him abusing uke too and since he's still so hang up on the guy, he might rally some forces to go against seme as well.
Yeah I too believe that becoming immune to poison was the main protective measure for people in high positions at that time. But if being interrupted at the precise moment he was about to take the poison is not a hell of a luck, I can't say anything else ╮( ̄▽ ̄)╭
This time the punishment need to be severe cause it involves a direct attempt to poison the governor, so if the order last time was death by arrow, I can't think of anything other than death either, except that he won't have the courage so the death could be faked.
Hope to be surprised by the author's solution.
--- Governor has a hell of a lot of luck.---
Any excuse from Yonjo won't fly now. The governor needs to set an example in the punishment, otherwise, how will he keep Yonjo by his side? I don't see any other way than to send him away.
So he can pretend to have killed Yonjo and send him into exile in some small, remote province? ╮( ̄▽ ̄)╭