Replying to close this out for anyone else who might encounter this in the future, Power BI caches the results from repeat calls to the same API. To work around this problem, you can add a unique value to the header each time you call out. Using the counter value already specified in the script is a good way to achieve this.
Credit to KenvM over on the Power BI forums for providing the solution.
Where to add the header value:
Json.Document(Web.Contents(scrollURL,[Content ==Text.ToBinary(scrollBody&scrollID&scrollEND), **Headers=[MyHeader=Text.From(Counter)]**])),