As pointed out by ofunny on the GeyserMC Discord, all you have to do is to create a local copy of $container_title.
Quote ofunny:
To solve this issue, just simply create a local copy of the variable, as it should be done, if the variable is being used to do comparison or any other manipulation of it.
"$fart": "$container_title",
"$ofunnymenu": "(('%.2s' * $fart) = '§')",
As pointed out by ofunny on the GeyserMC Discord, all you have to do is to create a local copy of $container_title.
Quote ofunny: