PG and VG are humectants and hygroscopic (have an affinity for water), they both attract and hold it, so throat build up is what to expect, and think of ot this way, your own body will actively produce flegm, this catches and holds foreign particles (a natural defense system), PG has anti bacterial properties, so this 'build up' you sense is similair (but an exagerated) form of 'first line defense' for your trachea.
If it seems excessive-the reasons might be, lots of vaping, humid cooler air, not enough flushing (drink water), or a dusty air (hence viscosity).
Nothing to be concerned about-just drink more water, and swallow oftener er.