After the 7.0.6 update my account was at 96%ish on /dexcheck. In the advancements page I had achieved up to 75% Completion. I filled in the remaining few I was missing. On the last Pokemon (Solgaleo, via a server pokesale command) /dexcheck read 100% completed. There was no chat broadcast, and when checking Advancements, it still read 75%.

Following this, I went out and caught a wild Magikarp to update it. This worked in updating Advancements, but there was no broadcast for it.

For any accounts that have already achieved 100% completion in previous versions, it is impossible to accomplish 100% Completion again, as it does not reset with the added Pokemon in later versions.

Thanks for the report. It sounds like there was some delay in you getting the advancement, but you did get it. As for the broadcast it could have been that your server has broadcasts off for that advancement, as we were able to get the broadcast just fine when testing.

Regarding the advancement not resetting when new pokemon are added, that's not a bug. The mod isn't meant to take away the advancement.