I'm hitting this problem with Org-mode 9.2.5:
Problem synopsis: When I use org-set-tags on a heading
with no tags, Helm gives me the correct prompt w/a list of all my tags.
After I have entered a single tag, then no more Helm suggestions are
offered. This is the bug, and it was fixed at some point.
Helm + org-mode *work correctly* when I run emacs using
the elpa/helm-20190726/emacs-helm.sh script which comes with Helm
for debugging purposes. I notice that the org-version in the *working* example
is 9.1.9 (which is, I presume) the stock org-mode that comes with Emacs.
I have reproduced this problem on both Linux and OSX.
Can anyone suggest a next step to take?