From what I've read, things started going wrong when an (ex?) official of the US released information the Dutch shared with their American allies[1]. Details on when or why access to the Russian networks was lost aren't readily available (secret service and all that) but I do remember reading about the intelligence agencies being not particularly pleased with the way the information was publicly released by their allies. It makes sense that someone gloating about how there's an ally of theirs in your network will make you take a second look at the traffic flowing through your network if you're waging cyber warfare.

[1]: https://www.washingtonpost.com/world/national-security/new-d...

In general, in the spyworld you do not talk about your tricks. But if you've already been found out, and your tricks no longer work, there is still political advantage to be had (damage your adversary, obtain credit from partners, obtain funding, easier recrutement).

https://arstechnica.com/information-technology/2018/01/dutch...

Or if you just want to claim it to damage your adversary, whether they have the unit you supposedly hacked and it does what you say it does, or not...

Covert access for over two years to every part of the GRU's election meddling apparatus is very impressive, actually. Once you're caught, too, there's sometimes value in sort of showing it off and embarrassing your adversary.

Or this is the least of their exploits, and releasing this info is a psyops thing.