When CREATE multiInput() it work fine with unicode characters. But when you UPDATE using updateMultiInput() it won't support unicode characters.
Create:
multiInput("testMultiInput", label = "Select Items", choices = c("小田", "若土"))
Evidence:
Multi Input show all Japanese characters

Update:
updateMultiInput(session, "testMultiInput", choices = c("123小田abc", "123若土def"))
Evidence:
Multi Input remove all Japanese characters

As I have checked, other input have same issue.
When CREATE multiInput() it work fine with unicode characters. But when you UPDATE using updateMultiInput() it won't support unicode characters.
Create:

multiInput("testMultiInput", label = "Select Items", choices = c("小田", "若土"))Evidence:
Multi Input show all Japanese characters
Update:

updateMultiInput(session, "testMultiInput", choices = c("123小田abc", "123若土def"))Evidence:
Multi Input remove all Japanese characters
As I have checked, other input have same issue.