Someone in the other comm had the same question and helpfully checked for more details!

So like…poison? Or did the king have some crazy double allergy?

Time for search engine rabbit hole

EDIT:

Gyeongjong suffered from ill health during his reign, and the Noron political faction pressured Gyeongjong to step down in favor of his half-brother, Prince Yeoning. In 1720, two months after his enthronement, his half brother, Prince Yeoning […] to handle state affairs, since the king’s weak health made impossible for him to manage politics.

It is said that, Gyeongjong’s mother, Lady Jang, was to blame for his illnesses. She was sentenced to death by poison, in 1701[…] she inflicted a severe injury to the Crown Prince’s [Gyeongjong] lower abdomen that left him sterile and unable to produce an heir.

There was some speculation from Soron party members that his half-brother, Prince Yeoning, had something to do with his death […] “But we may well doubt the truth of the rumour, for nothing that is told of that brother indicates that he would commit such an act, and in the second place a man who will eat shrimps in mid-summer, that have been brought thirty miles from the sea without ice might expect to die.”

Seems like a (hepatitis?) type of food poisoning, per Wikipedia. Crazy history though.

Seems like food poisoning that was used as an accusation of intentional poisoning.