Yep, the router/milling bit has a >0 diameter, so it’ can’t cut inside sharp corners.
For outward corners that’s not a problem, for inside corners you either have to file them manually or have the bit cut past the edge you want and cut in a bit into either side with 1/2 diameter depth, to get the material removed.
For 1.6 mm thick pcbs I usually use 1 mm milling bit diameter.
For 2 mm boards I make the ‘slots’ 1.5 mm wide.
PS: you can see how they adjusted the pad where the milling bit would potentially cut into the copper (=increased wear) and set it back a little (the PTHs are centered in the original pad).
PPS: that’s what I needed the sharp internal corner for:
One board slipping sideways into another one with no need for mechanical deburring/filing. Just soldering for the mechanical connection and done.