100gpd membranes usually aren't as efficient as a 75gpd membrane but getting a TDS of 49 from a Tap with a TDS of 350 is well beyond the normal drop in 100gpd efficiency. Even if say the temp was really cold and the pressure really low one should still expect over 90% rejection from a membrane with very slow production and at some point wont produce at all.
Great point! There still should be a reasonable expectation of rejection rate for a 100gpd membrane that doesn't really fit with @jeff williams reported numbers. Especially if that membrane is a DOW! It's pretty hard to speculate further causes without some more info.
-Randy