There are two reasons.
1, you hide the layer , please check the layer tool.
2, you maybe Hide it at Designer manager.
Please find help file via Layer tools and Design
I check your public project PCB
For any others who are also looking for missing traces/tracks...Go to menu selection Tools/LayerManager and change the transparency for the top & bottom layer. Selecting something like 50% made the traces/tracks appear again for me with the Layers and Objects selections.
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