Trying to replicate your issue in Lenses Box (a quick way to validate issues) failed.
If I had to guess, I would place my bet that the server or the container image —if you run things in containers— is not set to use UTF-8.
A little while ago, we saw a similar issue in Kafka Connect with a user on Confluent’s Kafka Connect docker images. Their images do not have a set UTF-8 locale (they default in C - ASCII by default), so some connectors had issues processing strings in UTF-8 unless we explicitly forced UTF-8 in the connector’s source code.
In short, I suggest you check that the server or container that runs Lenses and Schema Registry, both are set to use a UTF-8 enabled locale.