I found this which admittedly is not detailed and not the one I meant:
I hope that would have been qualified with “except for 4 hours if the ISP changes your IP”. And I find loads of user posts saying their dynamic works fine, and just a few where it didn’t, and those that I find are inconclusive. OK, many ISPs assign dynamic ones that however rarely change, if ever, but still I’d expect more uproar if it was a general problem.