Query with several consecutive LEFT JOIN - unexpected results

My goal is to populate several objects of several child classes (B, C, D, E, F...), all extending a parent class which is A. All of this, in one single and only big query.

Let's say I have several tables reflecting the classes structure, including these 3 ones:

Table A /* the parent class */
id       type        created
--------------------------------
392      B           1377084886

Table B /* one of the child classes */
id       myfield     myotherfield     anotherone    oneagain
-------------------------------------------------------------
392      234         'foo'            'bar'         3

Table G /* not part of the structure, just here for the query to check a field */
myfield      fieldtocheck       evenmorefields
------------------------------------------------
234          'myvalue1'         'foobar'

Now:

/* This (the query I want): */
SELECT
    a.*,
    b.*,
    c.*,
    d.*,
    e.*,
    f.*
    FROM A a
    LEFT JOIN B b ON a.id = b.id
    LEFT JOIN C c ON a.id = c.id
    LEFT JOIN D d ON a.id = d.id
    LEFT JOIN E e ON a.id = e.id
    LEFT JOIN F f ON a.id = f.id
    LEFT JOIN G g_b ON b.myfield = g_b.myfield
    LEFT JOIN G g_c ON c.myfield = g_c.myfield
    WHERE g_b.fieldtocheck IN (myvalue1);

/* Returns this (what I don't want): */
id->392
type->B
created->1377084886
myfield->NULL /* why NULL? */
myotherfield->NULL /* why NULL? */
anotherone->NULL /* why NULL? */
oneagain->3 /* why, whereas other fields from B are NULL, is this one correctly filled? */

Whereas:

/* This (the query I don't want): */
SELECT
    a.*,
    b.*
    FROM A a
    LEFT JOIN B b ON a.id = b.id
    LEFT JOIN G g_b ON b.myfield = g_b.myfield
    WHERE g_b.fieldtocheck IN (myvalue1);

/* Returns this (what I want): */
id->392
type->B
created->1377084886
myfield->234
myotherfield->'foo'
anotherone->'bar'
oneagain->3    

I have no idea why. Tried different things, but this is what I come up with. Has someone an idea?

EDIT: Clarified this post and made it more straightforward.