Not as epically as before:
In better news, it occurred to me that some of the groups (i.e. sites) might have distinct distributions. I previously explored the balance by month and day but not sites. Turns out there are small extreme values accounting for 2, 4, and 10% of the values in these three sites, respectively. Dropping these three sites, the model fits quite nicely in 200 iterations. So, as I suspected, something to do with the raw data. I’m still not sure why the truncated normal couldn’t handle all this, especially as in total these observations aren’t overly common, but posterior geometries are well beyond my understanding!