Both Espruino and RuuviTag use Nordic’s Flash Data Storage as a backend to store data for application. Something in the way RuuviTag stores data seems to prevent some versions of Espruino from booting.
For example I wasn’t able to run Espruino 2.06 on my RuuviTag before purge, and after purge I can. Hardware is the same, software is the same, only difference is the stored data in tag. DFU updates don’t automatically erase the user stored data.