I think the ballast detects rectifying then cut power if any. < 20W lamps are 60V and not 100V like most 4 footers, so the OCV is most likely high enough to force a rectifying lamp.
Based on the behavior you describe the EOL protection trigger is simply the arc voltage (or directly, or indirectly via hard switching detection, the later being cheaper, but less sensitive, so reliable for lower nominal arc voltages; but in order to protect the ballast itself and not e.g. sockets, the hard switching detection is far enough). So if you try worn out lamps, the one with 100V nominal would have actually 140V on the arc (so above the threshod), the 60V one would be about 100V, so below the EOL threshold, so do not trip it.