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Error: "No valid versions available for undefined" when installing from Nexus

I have a weird thing happending to NPM + Nexus.

Recently I set up a new Nexus instance (3.16.x) and added a npm proxy on it to cache the public registry.
For reasons I can’t pinpoint, some packages have no version information on them.
For example, this is the output of harmony-collections in Nexus:

harmony-collections from Nexus
    "_id": "harmony-collections",
    "maintainers": [
            "name": "benvie",
            "email": ""
    "dist-tags": {
        "latest": "0.3.8"
    "author": {
        "name": "Brandon Benvie",
        "email": ""
    "_rev": "70-78f571253790de44a2beee33730d11ba",
    "description": "Shim to provide Map, Set, and WeakMap if they're not available, with non-leaky O(1) lookup WeakMaps"

The same in the publich registry is much larger:

Is there any recent change or hiccup that would make Nexus pick up a partial JSON like the above from public registry and cause the issue?

The end result for is… When I run an install I get an error saying No valid versions available for undefined