I found the cause. There is a module that deep clones complex objects so they can be restored in the future. Can you spot the error in this recursive loop?

function cloneObject(src) {
    let target = {};
    target.__proto__ = src.__proto__;
    for (let prop in src) {
        if (src.hasOwnProperty(prop)) {
            // if the value is a nested object, recursively copy all it's properties
            if (isObject(src[prop])) {
                target[prop] = cloneObject(src[prop]);
            } else {
                target[prop] = src[prop];
            }
        }
    }
    return target;
}

When someArray goes through the cloneObject() function, it creates a new object called Array that does not retain true array properties but instead converts it into an object with proto='Array' (as @CRice pointed out) and nothing more. To fix this we really need to rework the cloneObject function to preserve arrays. This is the corrected deep copy cloneObject function I am now using:

function cloneObject(src) {
    let target = {};
    var isArray = Array.isArray(src);
    if(isArray){
        target = [];
    } else {
        target.__proto__ = src.__proto__;
    }

    for (let prop in src) {
        if (src.hasOwnProperty(prop)) {
            // if the value is a nested object, recursively copy all it's properties
            if (isObject(src[prop])) {
                var propertyValue = cloneObject(src[prop]);
            } else {
                var propertyValue = src[prop];
            }
            
            // if src was an array
            if(isArray){
                target.splice(prop, 0, propertyValue);
            } else {
                target[prop] = propertyValue;
            }
        }
    }
    return target;
}