Undefined

Anonymous you can try updating your Power BI Desktop to the latest version and see if that resolves the issue. If updating doesn’t work, try removing the month slicer and adding it again. Sometimes, this can help reset the slicer and fix any issues.

Another thing you can try is to check if there are any filters applied to your report that might be affecting the grid slicer.

You can do this by going to the “Filters” pane and checking if there are any filters applied to your report.
You can also create a new grid slicer and see if that works. Sometimes, creating a new slicer can help fix any issues with the old one.

Reference :
https://community.fabric.microsoft.com/t5/Desktop/Scatter-plot-not-showing-entire-data/m-p/946829

https://stackoverflow.com/questions/60754216/month-slicer-and-filter-not-working-properly-on-rolling-data-power-bi

Please let me know if this works.

Thanks