Hi,
There is a known bug with getMarketPrices which we haven't sorted yet - this is partly because the function is not really needed anyway (see next sentence) so it's not the highest priority.
getCompleteMarketPricesCompressed is the best function to use for getting prices. It returns more comprehensive data than getMarketPrices and there are no bugs with it.
However, note we are also planning a new release of the library and documentation this year, so any bugs/ ommissions should be fixed.