Most likely caused by 4c561ac

@cvNOliveira If you made a plunkr as requested in the github issue template I could have tested in 2 minutes and reverted the commit and resolved this issue. But you didn't 😞

Can you test reverting the change in the above commit and see if it resolves your problem? If it does, I'll revert in master. Thanks! 👍

You are right, my bad! I should have made the plunkr.
Anyways, I have managed to solve the problem excluding the theme from the condition on line 1388.
Probably not the best option, but it worked for me.

Old code:
if (!uisOffset(dropdown).height && $select.$animate && $select.$animate.on && select.$animate.enabled(dropdown))

New Code

var isSelect2Theme = angular.element(element).hasClass("select2");
if (!uisOffset(dropdown).height && $select.$animate && $select.$animate.on && $select.$animate.enabled(dropdown) && !isSelect2Theme)

I hope it helps.
Cheers!

Did you try reverting 4c561ac? I thought that would have been the cause... maybe not then.

I did not revert that commit because, as far as I can see, it has not to do with multiple choices and that commit is related with select-multiple template.

In this case, it's an opacity property that remains 0 because, for select2, at some point the animation procedure does not change the opacity to 1.
Since select2 does not have animation, I disabled the animation procedure for this theme as mentioned in my last comment.

Same problème 2 year after...
I've made a pr with @cvNOliveira solution, please take a look