Environmental data not working after flashing Ruuvitag

Got some odd behaviour trying to advertise some environmental data from a Ruuvi tag using Espruino. In a nut shell, flashing via web IDE and outputting env data works perfectly fine however as soon as I persist the firmware on the board the readings seem to skew and not change.

Can be reproduced with very minimum code e.g.

var Ruuvitag = require('ruuvitag');
Ruuvitag.setEnvOn(true);
setInterval(function() {
  console.log(Ruuvitag.getEnvData());
}, 5000);

Any ideas what's going on?