Hi Adrian,
Thank you for your post in the forum.
Per your description, we test on our side. However, we are so sorry that we couldn’t reproduce your situation based on your description, resulting in that we only can do below. Here are my reproducing steps for your reference. If you have any confusion, please feel free to post back, we will try our best to help you out.
First, we create a Pivot Chart, based on the Pivot Chart, click PivotChart in the Tools group under PivotTable Analyze tab > create a Chart > right-click the mouse in the Chart Area > Change Chart Type > change the Combo like your settings > click Slicer under Insert tab > select the aaa, bbb, ccc. For more details, you can refer to the screenshot below.
The two screenshots below are mainly to show the Chart changes when different Slicers selected.
After filtering the Slicer, the format of Average of aaa will still stay in Line format. I assume it may be related to the different situation, so I would like to collect your steps so that we can test on our side.
(If the steps are complicated, you can also share a sample file with us. Please remove the personal information and upload it to cloud drive, then share the access link with us. Thank you for your cooperation.)
Best regards,
Cindy