UNION with different data types can give wrong result

Consider this SELECT with UNIONs using different data types for the same column:

SELECT 20 AS x UNION SELECT null UNION SELECT 2.2;

Gives:

+------+

| x    |

+------+

| 9.9  |

| NULL |

| 2.2  |

+------+

It seems somewhat non-intuitive that 9.9 becomes 20 ...!

The issue was first reported here:
https://dba.stackexchange.com/questions/204757/problem-with-union-casting-integer-to-ceilingdecimal