This is what being dumped feels like. Only we don’t often see it coming. The dumper wants the relationship to end and the dumpee usually doesn’t. That’s what makes it so hard. Even if the dumpee agrees with you, the dumper is still getting what they want: the end to the relationship.
All the ways in which you described the dumpee being seemingly unaffected by the break-up is exactly how most dumpers act. And you wonder if they ever loved you at all since they could let you go.
The situation you described sounds like the dumpee is the kind of person who waits for their partner to end things so they themselves don’t look like the bad guy. So really, who’s dumping who in that scenario?
Ending a relationship can suck on both sides but, in my experience, breaking someone’s heart isn’t as devastating as having your heart broken. Ending something you want to end isn’t the same as having the rug pulled out from under you. Dumping someone doesn’t usually leave you questioning your worth as a human being, where being dumped can.