Of course they are fungable. The metric for making a payment is sats per vb and the protocol could give a shit less which sat it is that pays he fee.
People can put arbitrary value on whatever they want. You can have the same problem in any currency. Dollars are fungable. But the ones that say silver certificate are worth more than a dollar.
But go to dollar general and try to tell that to the cashier when she rings you up and says it'll be $10 and you say oh but this dollar is special and actually worth $10. Neither the cashier or dollar general will give 2 shits about the story you wanna attach to that dollar.
