SET max_statement_time= 2; # Just so that if it hangs, then not forever

DROP TABLE IF EXISTS t3;

CREATE TABLE t (c CHAR(8), KEY(c DESC)) ENGINE=MyISAM CHARACTER SET utf8mb4;

INSERT INTO t VALUES (''),('foo'),(NULL),(''),('bar');

SELECT DISTINCT c FROM t;

SELECT c FROM t;

# Cleanup

DROP TABLE t;

The first query returns three values, without NULL:

SELECT DISTINCT c FROM t;

c

bar

foo

If count is added, it claims there are 3 empty strings instead of 2.
The second query, plain SELECT without DISTINCT, hangs (subject to max_statement_time).

Reproducible with MyISAM and Aria. Not reproducible with InnoDB.
Reproducible with big5, utf8mb4, utf8mb3. Not reproducible with latin1. I didn't try other charsets.
Not reproducible without DESC index.