[SalesForce] Flow calling Apex, can’t pass Collection Variable

I have an invokable apex method that I need to call from a Flow. Invokable methods are strict in the data type of their parameters — I can't simply pass a string, instead, I have to pass a list<String>. OK, no biggie. Here's the method:

    public class ARpromptFirstName
{
    @InvocableMethod(label='Send AutoResponse FirstName' description='Needs ConversationIdList')
    public static void sendAutoResponseFirstName(List<String> ConversationIds)
    {
        for (String Id : ConversationIds)
        {
            String msgBody = 'Please enter your first name.';
            SaveSendSMS.sendSMSfromLightning(Id, msgBody);
        }
    }
}

The method works fine when I call it from Anonymous Apex. My problem is calling it from the Flow. The method shows up in the Flow palette, and I can drag it into the Flow, but it won't allow me to assign a collection variable to the ConversationIds parameter. It gives me the choice of all my other (singleton) variables, but not the collection variable named ConversationIds to which I have added the individual {!ConversationId} I need to pass. I think it's odd that it allows primitive data types to be passed to a parameter that is a list<string>. (And, no, the method won't work if I just pass a single string value instead of the list.) That suggests a breakdown in communications between the APEX method and the Flow Designer.

YES, the flow variable is set to Output (I've tried Input/Output as well).

Checking the documentation, it says:

Pass information from the flow to the invoked Apex method. The method
determines the available input parameters and their data types.

As you can see in the method code above, my method clearly indicates list<string>. The flows editor doesn't allow you to be that clear when you create variables. I've created a "Collection Variable" of type "Text". I think that's about as close as one can get to a list of strings.

A day later…
I suspected that the flow's Collection variable of type Text isn't exactly a list<String>, so I've tried changing the method to accept a list<Conversation__c> parameter instead, creating an SObject Collection variable in the Flow. But guess what… that doesn't work either. Only the singular variables/sObject variables appear in the pulldown menu of items that can be assigned as the parameter when calling the APEX method. Frustrating… Flow can't send a list, APEX can only accept a list.

I've combed the docs for Invokable Apex and for the Flow Designer — can't find anything I've overlooked. I'm hoping one of you can spot something… Thanks.