The investigation started last year, I'm guessing like with some darknet markets, the Dutch police decided they'll run the service themselves for a while to glean some information from people using the service. Or they managed to locate bestmixer's servers and install a backdoor.
I was wondering about that too,
the service started to operate in
march 2018 as we can clearly tell by the OP date
then I see under the seize warning, dutch law enforcement has started investigation in
june 2018 so only 3 months after its operation, they already suspicious of this service?
maybe the creator of this service has been in the watch-list of dutch cyber police from the beginning