Illegal mix of collations

Inconsistent collation treatment for some function for user variables when comparing values.

Receiving error Error Code: 1267. Illegal mix of collations (latin1_swedish_ci,IMPLICIT) and (utf8_general_ci,COERCIBLE) for operation '=' for some operations that previously worked on 10.3.23, but now fails on 10.5 (10.5.1 ... 10.5.4).

Comparisons for functions tested so far that fail with the above error:

SET @x:='dummy';

SELECT @x:= 123456789, @x = BIN(@x);

SELECT @x:= 123456789, @x = FORMAT(@x, 4);

SELECT @x:= 123456789, @x = JSON_QUOTE(@x);

SELECT @x:= 123456789, @x = MD5(@x);

SELECT @x:= 123456789, @x = SHA1(@x);

SELECT @x:= 123456789, @x = SHA2(@x, 512);

SELECT @x:= 123456789, @x = FORMAT(@x,'#,###,###.##');

Importantly, this will work if the first line is SET @x:=1;. Also, if @x = is removed, they also work.

I suspect that this has to do with the collation of user variables acting differently in v10.5, and may be related to MDEV-21815 [#https://jira.mariadb.org/browse/MDEV-21815].