Why don't give the phone a hard reset as it helps clear any hiccups if there exist in your phone specifically and brings it back to the default OS. You will have to start from scratch but at least you can be confident that the possibilities of hiccups is zero....unless there are bugs which means you need to wait for an update to resolve it.
I would forward the solution to KaiOS Technologies website. I'm sure they will forward it to the proper software engineer as they handle the OS and may even forward it to their reference man in China, who knows.
The KaiOS HQ is in Hong Kong so if they don't test the Chinese input then that's very worrying...