UNHEX() of a somewhat complicated CONCAT() returns NULL

I have a SQL script that works on MySQL (up to current versions). I built a copy of MariaDB-10.2.8 from source on a Linux machine and got the database up and running. However, my SQL script fails, because MariaDB is returning NULL from an UNHEX() call, where it shouldn't.

The call is producing a 20-byte random binary string in a particular format (it's a BitTorrent node ID). I concatenate some required bytes with some random bytes, with certain bytes being limited to a particular range of values. These are constructed as a 40-character hex string, which I then run through UNHEX().

The SQL is:

unhex( concat( '414C2',

                        hex( 8 + round( rand() * 7 ) ),

                        substr( sha( uuid( ) ), 6, 33 ),

                        hex( 2 + round( rand( ) * 8 ) ) ) )

Running this (with select) returns NULL. Running just the concat() returns a hex string, and running unhex() on the hex string presented as a literal returns the expected binary gobbeldygook.