I think it happens more than you realize.
I'm not just talking about going to the conference finals or further in one season and then losing in Round 1. That is a pretty common thing.
You've listed a bunch of examples where teams still win 50+ games and get HCA, but have a disappointing Round 1 in a tough Western Conference.
That's quite a different scenario than this Celtics team.
Portland didn't win 50+ games and get HCA. They were the 8th seed with a losing record.
The 07 Heat won 44 games and had the 5th best record in the EC.
The Mavericks in 12 were the 7th seed at 36-30 (44-45 win pace). The 04 Mavs did win 52 games, but were the 5th seed when they flamed out.
The 18 Spurs were the 7th seed, though that was the year Kawhi got hurt so not exactly the same core. The 2000 Spurs were the 4th seed, and lost to the 5th seed Suns that had the same record as them.
Now sure a couple of those Spurs and Mavs teams were very good and got majorly upset in the 1st round, but as shown above that wasn't the case for most of those teams.
And of course the other side of that equation is those teams, by and large, were all making the Conference Finals as upper 50's or 60+ win teams. That wasn't the case with Boston. Boston overachieved in the playoffs quite a bit based on its regular season record.
I also didn't mention other teams like the 15 Rockets that won 56 games and made the WCF, and then turned into a 41 win team 8th seed and 1st round loser in 16 (they lost to the Warriors both times). The Thunder won 59 games and the WCF in 14 and didn't even make the playoffs as a 45 win team in 15, though Durant did miss a bunch of time in 15. The 10 Magic were in the ECF, and then were the 4th seed and a 1st round exit in 11 (though they did have quite a few more wins than the 5th seeded Hawks, so that was obviously an upset).
Again it happens a lot more than you realize.