Coins used to be largely worth what they were worth because of what they were made of. So cutting them in half would lead to two seperate halves of the value. I'm sure there's more nuance than that but broadly I think that's how it used to work.
This is not so much a gradual change, as a recurring theme in coinage through history, with the stability and grasp of a regime/kingdom/empire being related to how much seigniorage (profit) it can currently extract from the economy by putting the leader's face on a costly bit of metal. This may be from one of many periods where the old coinage of an obsolete regime reverted to holding negligible value over its metal value.
