I can understand that defensive rigidity from one season to the next isn't the easiest to maintain. We only conceded 22 goals in 38 games last season. The stat about how good we were (for most of the season) in keeping the lid shut in the final 15 minutes. We were so resolute in those areas defensively that trying to repeat either of those was likely asking too much. Defense requires so much mental focus, staying switched on at all times game after game, tracking runs, holding the line, all that kind of stuff. It's difficult to maintain a high level year-over-year.
The much bigger problem I'm trying to figure out is that we have exactly the same players in our defensive ranks this year that we did last year. There were no signings, no departures. It's all the same players. So why does the chemistry seem so off compared to last year? Chemistry shouldn't be something that degrades with time. This might be the most frustrating thing to me.