blocks are moved to another layer
When grouped blocks are moved to another layer and then ungrouped, they return to their original layer
This is a Feature of InDesign and not a bug.
If you want to stop this from happening, go to the Layers panel flyout menu and uncheck the option called “Ungroup Remembers Layers”. When this option is unchecked, the ungrouped items remain on the same layer as they were (when grouped) and will not return to their original layer.
Hope this helps.
Adobe InDesign team
Christophe Zeller commented
Thanks ! It works :)