It often depends on the router, and also of course on the normal night-time dip amount. Some routers will even hold connection at -1dB.
What has probably happened over time is that bit-swapping has occurred to the level that certain frequencies/bins have been ruled out by the router for effective use.
As the other bins' spare space, (their margin) is swallowed up by the bit allocation swapped in to maintain the sync speed, the average noise margin across them all, which is we see at this level, has to fall. Once it does that they don't come back into service until a router re-sync.
And if there isn't sufficient spare capacity in any bins to do any more bit-swapping, that's when you get a disconnection.
