There is a problem on all platforms; to varying degrees.
Windows 32bit and Windows 64bit:
When supplied BIGINT UNSIGNED, the return type is a BIGINT SIGNED,
The return type appears to be valid however for BIGINT values, the comparison is always
performed as if the values were signed.
Workaround would be to use IF(x>y,x,y) function instead.
How to repeat:
To reproduce this bug, create this simple table.