SortKey becomes empty on update when using nested values in default function

[Jroon94]

So, I have my secondary index's sort key set up as follows:

sk2: {
  hidden: true,
  onUpdate: true,
  default: ({ metadata: { firstName, lastName } }) => `${firstName}#${lastName}`,
},

When using Entity.put it populates the sk2 as expected,
However if I call Entity.update to update any attribute sk2 will become empty. This happens even if the metadata attribute is untouched.

If I move the firstName and lastName attributes out of the metadata map and assign them as individual attributes the sortkey is updated without issue. Is using nested attributes in default not supported? Or am I perhaps missing a step somewhere that makes this possible?

[Jroon94]

Update: This only happens when using the $set operator. If I pass the entire metadata object into the update function, the default is triggered and updated accordingly.