The UUID function returns its value in the UTF8MB3 character set. Since the
default character set has changed to UTF8MB4 in 8.0, a conversion needs
to happen, so Item_func_uuid is replaced by an Item_func_conv_charset
which will call Item_func_uuid::val_str. But, since Item_func_uuid has
const_item_cache=true by default(it is set by Item_func), the calculated
value gets cached and returned for each row.
Solution is to override const_item() to return false so that
Item_func_conv_charset sets use_cached_value to false in the constructor
and a unique uuid is calculated for each row.