Problem using brush tool on an inverted layer

I have a problem that only started recently. When editing photos, I drag the "background" layer in the bottom right down to the + sign to make a copy. I then select, Filter - Other - High Pass and then change it to "overlay". I copy this layer again. Select both "Background copy" and "Background copy 2" and click the folder icon in the bottom right to group them, becoming "Group 1" in the bottom right. At this point, both high pass layers have been applied to the image. I then hold ALT and select the "Layer Mask" button in the bottom right to invert. 

The problem arises at this point. I used to be able to select the brush tool at this point and paint in whatever I wanted the high pass overlay to be applied to in the image. However, now when I select the brush tool, it won't let me paint anything. Is it possible I changed some setting by accident? I have been able to do this the past few years with no issue.

If I don't invert the layer mask by holding ALT, and instead click "layer mask" button while selecting "Group 1", it allows me to paint using the brush tool.

Thanks.