You can successfully edit annotation and see it while switching to other nodes in tree and back again. But once you save the template and open it again - the specified annotation is gone.
If you manually edit *.oet and put:
... annotation="some-text-here" ...
to that node you can then open *.oet in Template Designer and see it perfectly well. But save the template one more time - and the annotation is gone again.
2.6, 2.7, 2.8