I went ahead and compared the .zarray files of my two datasets to be sure that there wasn’t some data type nonsense going on.
{
    "chunks": [
        50000,
        290
    ],
    "compressor": {
        "blocksize": 0,
        "clevel": 5,
        "cname": "lz4",
        "id": "blosc",
        "shuffle": 1
    },
    "dtype": "<f4",
    "fill_value": 0.0,
    "filters": null,
    "order": "C",
    "shape": [
        4702488,
        290
    ],
    "zarr_format": 2
}

{
    "chunks": [
        50000,
        290
    ],
    "compressor": {
        "blocksize": 0,
        "clevel": 5,
        "cname": "lz4",
        "id": "blosc",
        "shuffle": 1
    },
    "dtype": "<f4",
    "fill_value": 0.0,
    "filters": null,
    "order": "C",
    "shape": [
        477285033,
        290
    ],
    "zarr_format": 2

Everything looks identical except for the number of rows.