Its not an issue at all, the wave soldering that is used to solder the through hole components on the bottom of the board also sometime leaves some solder on the vias used to transfer the heat. We may mask them in the future but it does not effect the performance.
A few points on solder in thermal vias:
It’s bad if the vias to suck in solder from the top side. This can cause bad joint to the thermal pad in the bottom, voiding the whole point of the thermal connection. This is the matter of designing the via pattern and solder paste pattern to minimize the possibility. Via size matters.
If the board goes to a wave, it is good if the vias suck in solder from bottom. This much improves the thermal flow.
You don’t want to cover the vias from bottom and leave top side uncovered. This creates pockets of air under the chip, possibly covered with solder paste or melted solder from the top. All kinds of funny effects (up to exploding chips) can result.
You can tent the vias on both sides, but tented vias for thermal pads are tricky.
And folks at Duet3D know all this already. I’m just commenting out from my manufacturing experience why it is made the way it is, and why I think it is according good engineering practice, even though it might not always look pretty.