Hi thank you both for an amazing response!
I have updated both the plugin and Discourse to include latest changes, but issue with user profile page omitting drop down field remains for me.
A possibly related issue is that in all cases, I have to click twice on the User Profile link on left hand side of User preferences page. The first click brings up the standard user fields only, and only the second click brings up the custom user fields as well - but excluding the multi-select field (unfortunately, clicking 3 times does not help)
I have tried on several browsers, and cleared browser cache in each case
I can confirm that new user signups can now see the drop down options however.
Does this narrow the problem down?