This isn’t a new landscape malpractice but one as old as dirt. Wherever there is an empty space of dirt that sees the light of day, Mother Nature will fill the vacuum with plants. And they are usually weeds. That goes for sparse turfgrass, unmulched landscape beds, areas with not enough mulch, and even a cement sidewalk crack with 150,000 grains of sand (one-half square inch).
The proper landscape maintenance before applying a new layer of mulch is to remove all the weeds, level the ground, and then put down the mulch. Depending on how much mulch is already in the bed, the landscaper would only put down enough mulch to bring it up to 3” – 4”. Mulching over weeds is an endeavor in futility. The weeds will only flourish with 1” to 2” of mulch and spread. Eventually the weeds take over, need removed and it needs more labor to remove around the already existing shrubs and groundcovers.
Don’t mask the weeds. Clean the bed out properly, remove all the weeds by hand or by chemical, (preferably before they have gone to seed), and then put down the mulch. Less work in the long run. Your landscape beds will look well-maintained and attractive for a season or more.