I have plenty of experience of a dim charge light which glows brigter under load. In 1976 my Dolomite did this and alternators were £25. A lot then, so a piece of black tape over the light fixed the problem until I could afford a new alternator.
Those Lucas ACR units do vary slightly but I was told sometime later at an auto electrical shop, that the charging circuit is distinct from the warning light circuit. Both are diode controlled, and I recall he said that some charge circuits had three and the warning lamp circuit two. So it was quite possible to have charge and also have a glowing light which glows brighter under load when the diode has failed.
I have no idea if this is still the case. Maybe now a circuit board which replicates the old set up, but it's the "diode" in the warning lamp circuit, almost certainly.
My only concern with this theory is that the old alternator had this problem, so I replaced it with brand new and the problem continued straight away.
