https://clickhouse.yandex/reference_en.html#Data%20types
FixedString(N)

When reading a string that contains more bytes, an error message is returned.
When writing a string, null bytes are not trimmed off of the end of the string, but are output.

I assume this should be:

When writing a string that contains more bytes, an error message is returned.
When reading a string, null bytes are not trimmed off of the end of the string, but are output.

Regards,
Oli