The 23.6 MHz band is particularly prone to noise ingress (I think some DAB radio transmissions are in this range, so all it needs is a slightly dodgy cable joint somewhere). However DOCSIS is doing exactly what it is designed to do, which is, if necessary, downgrade a particular channel so to at least maintain a good connection rather than try to keep it running at 'full' speed and risk data corruption.
In theory, yes a degraded channel does imply a slower speed - BUT, the throughput of the channels as they are now, is far more than you are signed up to anyway. As you have found, a QAM drop on one or even two channels, probably doesn't cause any real-world issues at all.
Think of it like a pothole in the road, it's quite small and doesn't stop you from driving at 30 mph over it - it might be an issue at 90 mph, but you aren't allowed to, indeed can't, drive that fast down the road anyway - so is the pothole actually a problem?
