Works for me on my development environment. Can you provide a failing test?
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I will try it. But for now I'm using this method for workaround and detection:

'# Overrides Redmine::Acts::Customizable::InstanceMethods#available_custom_fields
def available_custom_fields
if !(Project.find_by_id(self.project.id).issue_custom_fields - self.project.issue_custom_fields).empty?
debugger
puts "Found this bug for project: http://www.redmine.org/issues/13118!"
end
if !(Tracker.find_by_id(self.tracker.id).custom_fields - self.tracker.custom_fields).empty?
debugger
puts "Found this bug for tracker: http://www.redmine.org/issues/13118!"
end
#workaround for this bug:
this_project = Project.find_by_id(self.project.id)
this_tracker = Tracker.find_by_id(self.tracker.id)
retval= (this_project && this_tracker) ? (this_project.all_issue_custom_fields & this_tracker.custom_fields.all) : []
return retval
end

Now I have detected the problem also once for trackers.
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Thomas Kohler wrote:
> This means "a_issue.project.issue_custom_fields" and "Project.find_by_id(a_issue.project.id).issue_custom_fields" give sometimes different results.

@a_issue.project@ and @Project.find_by_id(a_issue.project.id)@ are not the same objects in memory. You can try:

a_issue.project.object_id == Project.find_by_id(a_issue.project.id).object_id

So if you update an association (eg. add a tracker or a custom field to a project), you may need to @#reload@ your issue in order to see the new association.

I'm closing it, please reopen if you have a valid failing test.
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Yes, you are right! I have given that the a_issue.save do that already, but unfortunately not.

I have finished a failing test now and also a test with your solution. In my opinion a #reload in the method would be a good idea. Or inside the Issue.save? What do you think about?

def available_custom_fields
if !(Project.find_by_id(self.project.id).issue_custom_fields - self.project.issue_custom_fields).empty?
self.reload
end
if !(Tracker.find_by_id(self.tracker.id).custom_fields - self.tracker.custom_fields).empty?
self.reload
end
retval= (project && tracker) ? (project.all_issue_custom_fields & tracker.custom_fields.all) : []
return retval
end
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Please check my failing test and post your opinion about "reload in the #available_custom_fields or on/after #save of the issue".
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