In MariaDB 10.3.2, instant adding columns for innodb is supported.
After instant adding columns, the inplace update trigger assertion in some cases.

How to repeat:
create table t1 (c1 int primary key, c2 int);
insert into t1 values(1,1);
alter table t1 add column d1 varchar(20) not null default 'aaa';
update t1 set d1 = '' where c1 = 1; – assert in rec_set_nth_field()

The length of "d1" stored in the record is 0, because of instant adding columns. After update, the length is 0 also. But it can't do inplace update here.

So, it need to do some special check for rec_offs_nth_default() in row_upd_changes_field_size_or_external();

The patch can be work for it.