Even if it is cumulative shouldn't it only need RSIperiod worth of data to calculate it's first value. At worst I could see it requiring 2*RSIperiod so it can step over a full window to get rid of any 'noise' in the early values.

Again my coding experience lies mostly in R and MQL4 and this kind of behavior is not seen there. I'm just a little confused

Is there some way to calculate what an indicators lookback requirements will be relative to it's settings (so that I can deal with this when it comes up again)?

Cheers,
BobbyT