What brand of NaOH are you using?

Have you tried a clean evap with the shelitte, to see if it evaps cleans?

Sounds indeed strange - but to be 100 % sure maybe try a pH paper.

Let your Naphtha run through a coffee filter first also, to make sure whatever you get a the end was truly dissolved. Then evaporate Naphtha and check what happens if you put some water droplets on that remainder. Should be deep (green-to-)blue, while Alkaloids would only be green (if they would even dissolve in water)