org.hibernate.dialect.MySQLDialect#getCastTypeName:

		switch ( code ) {
			case Types.INTEGER:
			case Types.BIGINT:
			case Types.SMALLINT:
				return "signed";
			case Types.FLOAT:
			case Types.NUMERIC:
			case Types.REAL:
				return "decimal";
			case Types.VARCHAR:
				return "char";
			case Types.VARBINARY:
				return "binary";
			default:
				return super.getCastTypeName( code );

The mappings to "decimal" are incorrect. The problem is that on MySQL and MariaDB DECIMAL (no precision/scale) is DECIMAL(10,0). So any value being cast that have digits to the right of the decimal point will have those digits dropped (12.399999999994 becomes 12).

We need to given precision/scale to the result here. The problem is what values to use for precision/scale?