Your understanding is correct... I don't know any reason why it would fail to apply on Apo other than him resisting it, but then that should have been shown. At a minimum, it's a visual bug failing to show the resist, or it could be another kind of bug. 

Not a bug. Your Umbral hit the Apothecary for more than 25% of his HP and activated his Resurgent mastery- so the provoke was essentially removed simultaneously as it was placed. 

Usual Umbral Enchantress builds avoid crits to avoid this from happening. 

Thank you L9753 and Lxzy, I will alter my Umbral to avoid crits.

Does the Resurgent mastery not display a "Remove Debuff" message when a counter-attack is activated?

I noticed the message occurs when a debuff is removed without a counter-attack but fail to see any "Remove Debuff" message with a counter-attack.

Below are examples of what I mean, in addition to the prior posting with the Apothecary.

The first, shows Madame having a debuff removed when taking 25%+ damage. At 27 seconds into the video, there is a "Remove Debuff" that scrolls up. https://youtu.be/kjqxulRDNvw?t=26

The second, shows Martyr doing the same as the Apothecary in the prior posting, and again, the only messages that scrolls by aside from the damage amount are "Counterattack". https://youtu.be/_0B5o5FiziM?t=24

It is a little confusing to know what happened when the text message is displayed in one instance, and not another. I had assumed the Provoke didn't activate, as no message is displayed when a champion's skill does not activate.

I do appreciate your reply and hope my slight rant does not imply otherwise.

Thank you,