Hi,

First observation:
>
> When I set the heading 1 to DONE (without assigning it any other
> keyword), the TRIGGER events are ignored totally.  I guess this is
> an edge-case that may be considered as a bug.
>

​This is the result of Line 219 in org-depend.el:

(unless (and (member from org-not-done-keywords)
            (member to org-done-keywords))

In this instance 'from' is set to nil so the 'and' returns nil and
org-depend doesn't process the change of TODO to look for dependents. I
don't know why it was defined this way as org-todo.el accepts headings with
no TODO entry. I can't think of any reason other than explicitly blocking a
user from processing a heading without a TODO entry, but it seems that if
the user went to the effort of populating a TRIGGER property that they
probably want this to be acted upon by org-depend.el.

If there are no concerns raised in this thread I'll push a patch to all
have org-depend process the example you listed.


> Second observation:
>
> However, what is bugging me even more is that even when "heading 1"
> has a TODO keyword assigned, in *some* cases, the TRIGGER event does
> not get executed when I do it in my large Org-mode file.
>
> Copying the corresponding headings (the one with the TRIGGER prop
> and the headings containing the "target" IDs) from my real-world
> Org-mode file to my small test.org from above and marking the
> heading that contains the TRIGGER properties to DONE, it then works
> as expected.
>
> I then took the simple example from above, added a TODO keyword to
> the heading 1, copied it to my real-world Org-mode file and even
> this did not work: Heading 2 and 3 don't get their "NEXT" keyword
> assigned.
>
> So the behavior changes within different Org-file contexts.
> Therefore, I do have an issue creating a minimal example to
> demonstrate the bug.
>
> Can somebody give me an advice how to debug this behavior?
>

​My only thought is that there is an ID clashing with your sample ID and it
is being updated instead of the heading you are interested in.
org-depend.el looks at the local file for an ID and then looks globally for
an ID match.

You could instrument 'org-depend-trigger-todo' to trace the behaviour while
processing the TODO update.

Otherwise, you could use a process of elimination in a copy of your
existing file to find the MWE that causes the issue and post it here. You
could successively delete headings or other entries until the issue stops
occurring.​

I have read your post on org-depend and found it insightful so thanks for
that. When I was looking around at patching some new functionality into
org-depend I was put onto org-edna[1] which was built as an alternative to
org-depend. The syntax isn't compatible with org-depend, but org-edna is
more powerful than org-depend. I am now using org-edna and am quite happy
with it.

[1] https://elpa.gnu.org/packages/org-edna.html