Rocksdb text prefix indexes return wrong results

Rows are missing from the output when searching by indexed text column when the value is longer than the index's prefix length. Using "like 'prefix%'" only works when the prefix is shorter than the index's length (or equal, if value=prefix).
Deleting the index, using ignore index or converting the table to another engine gives expected results.
The issue seems to only happen when using utf8/utf8mb4/utf16/utf32 collations, latin1/ascii/ucs2 collations work correctly.

create table tt5 (tx text) engine=rocksdb;

insert into tt5 values ('long text');

select * from tt5 where tx='long text';      # returns the row

create index z on tt5(tx(3));

select * from tt5 where tx='long text';      # empty result

select * from tt5 where tx like 'long text'; # also empty

select * from tt5 where tx like 'lon%';      # empty

select * from tt5 where tx like 'lo%';       # this returns the row again