I assumed the funds were lost, no problem, my fault. I was just reporting a possibile issue related to a chrome update. But at the end I'm actually more interested in how the mint can manage this "vacant" funds. Can it somehow "unlock" them, for example when it see that the tokens are not spent for a long period, or it have to shutdown completely? Are there any default policy about that?

only after rotation and deactivation. we don't have a default policy but we've built everything that's necessary for it. mint deactivates a keyset, and roatates to a new one. wallets detect this and automatically rotate funds to the new keyset when they move their coins. a long time after giving everyone the chance to move, the mint then rugs the old inactive keyset. this is also how the mint keeps its database size constant over a long time period.