Visulisation issue when using 6 month rolling average measure and relative date slicer

Hi, I'm looking for some help with one of my reports.  I have some bar and line graphs that use a 6 Month Rolling Average measure for the line graph.  Due to the way my data is set up there are entries up to the end of 2024, however to focus on the most recent values that have occurred within the last 6 months I then put a relative date slicer on the visualisation for the 'Last 6 Months'.

However when doing this December 2023 is present on my visualisation, which clearly isn't correct.

In trying to understand why this was happening I found that if I removed the 6 Month Rolling Average measure the graph showed the true last 6 Months.  

I also checked if smaller figures changed the outlook and worked out that if I set the relative date slicer to 'the last 9 weeks' December does not appear.  Since this check was carried out in Week 10 of 2023 I can only assume that when I use the 6 month rolling average measure to look back and the time period is over the change in calendar year that this is causing an issue?  (please note that this was not happening last year)

Does anyone have any ideas how to sort this?

I've anomolised the above graph but can't post any files.  I have a date table linked to my input data, and my 6 month rolling average measure is doing the following: