Okay, I think the issue is that /mapped-types is reading the metadata storage object from class-transformer/cjs/storage whereas when the parcel bundler is importing class-transformer it's importing the esm5 version of the package class-transformer/esm5 so the MetadataStorage object, when Nest is trying to read it in the browser setting, is empty, there's no metadata in the map.
IF class-transformer were to use the Reflect API and store it directly on the object, this could be avoided pretty easily, but as that's not the case, we'd have to figure out some way to determine if class-transformer should be imported via a require statement or the esm import.
The other possible solution would be to have all of Nest's code use ESM, which is an eventual goal to align with the ecosystem, but it's not a huge rush at the moment as tooling still isn't just great around it, and it would be a rather large breaking change to existing code bases