Thanks for the additional info Father Damien-Marie.
Do you happen to know what temperature is inside the room where you have your plant? Northern exposure light is rather weak, and it is good you have some extra lights there. Typically plumerias just go dormant during winter and colder temps. So it is natural for the plant to just drop off their leaves and any blooms it may have at that period. Unless you are really able to maintain the temperature, humidity and light needs it has, some growers who have climate controlled greenhouses can do it but it is a mixed thing..sometimes just got to respect the plant's dormancy.
They just resume more active growth again, when temps start to go warmer when overnights no longer go down to below 50F, and daylight hours go longer.
What is the condition of the entire stem, is it feeling firm..any soft spots below soil line? I would also check that part just to make sure it is not beginning to rot there, hence unable to sustain any new growth above.
As for the mites, I think just got to be patient with those pesky things..neem oil is a good remedy which you are already applying. Or maybe change the media as well..maybe it is just too spider mite infested as well, so it just keeps coming back.
