The Create Poll endpoint has optional query parameters to allow additional votes using either bits or channel points.

However, it doesn't actually allow multiple votes. This can be seen in the pubsub response which explictly states that "multi_choice" (property indicating whether additional votes are allowed) is not enabled.

When creating a poll through the web interface, it works just fine as it is created through the GQL and not the API. The GQL message also has an additional property to explicitly allow additional votes, on top of the properties that are used to either allow additional votes by bits and/or channel points.
So either the query parameter is just not documented... or the endpoint itself is flawed because like half of its functionality is just broken.

Also gonna drop the documentation link as a reference: https://dev.twitch.tv/docs/api/reference#create-poll

Feel free to hit me up in case you want additional information regarding this (or when I just overlooked/misunderstood something)