When using blockstates to randomize block models, using a blockstate file that randomly selects between two models selects the first model 100% of the time even though it should be a 50/50 chance for each. Adding "weight": 1 to the second model allows it to be included as a workaround. Blockstates having three or more model options do not experience this issue. In this example, only the model for stone will be used even though dirt is included. My best guess is that the second model is given a weight of 0 unless otherwise specified.

{
    "variants": {
        "normal": [ 
            { "model": "stone" }, 
            { "model": "dirt" }
        ]
    }
}

edit:

This has been fixed for most blocks, however blocks such as flowers still exhibit this behavior.