Since JSON has no URL data type, for JSON-LD, either the context has to be clear that a property expects a URL, or the property's value has to be an object with

or whatever. This is mostly right in the JSON-LD API, but there are a few fields that sneak through. I've just noticed this issue for albumReleaseType and albumProductionType. Arguably, this is a schema.org issue (on the assumption that they should be declared as requiring particular schema classes), but the easier fix might be just to change the API code?

[Note of caution: I'm not super fluent with JSON-LD, especially contexts, and I only spotted the errors when I converted the file to turtle, at which point the strings-instead-of-URLs issue jumps out]