Why does Groovy change type of my collection?

I use groovy 2.5.8. I’m stuck on a problem:

1. I create def param = [:], and I check in my logs that it was created only like a map (line 219)
2. I send this def to a method, where the map must become filled. I also check, what type of def will be sent as a response of the method (line 176)
3. I check type of the def param again, and it become ArrayList…

Have somebody stuck on the problem? How to fix it?

Here is parts of my code:

def checkActivePurchase(RenewIviResponse response) {
    response.result.purchases.each {
        def finishTime = ZonedDateTime.parse(it.finish_time)
        def timeWithinMonth = ZonedDateTime.now().plusDays(30)
            if (finishTime.isBefore(timeWithinMonth)) {
                def params = optionPurchase(response)
                log.info("Line 176, params were requested, sent params   class is " + params.getClass())
                return params
            }
    }
}
    
def option(def response) {
    def params = [:]
    if (response.result) {
        if (response.result.purchases) {
            log.info("Line 219, requesting params, current params class is " + params.getClass())
            params = checkActivePurchase(response)
            log.info("Line 221, got params class is " + params.getClass())
        }
    }
}
    
def optionPurchase(RenewIviResponse response) {
    def params = [:]
    def result = response.result
    def purchase_options = result.purchase_options
    purchase_options.each {
    if (it.product_identifier == iviOptions.product) {
        def paymentOptions = it.payment_options
        paymentOptions.each {
            if (it.purchase_params?._product_identifier == iviOptions.product) {
                def purchase_params = it.purchase_params
                purchase_params.each {
                    if ((it.key as String).startsWith("_") || it.key == 'sign') {
                        params[it.key] = it.value
                            }
                        }
                    }
                }
            }
        }
        params