Cannot place external file inside a Block in Rhino 6

I am not able to insert an external file as a block inside another block.
Normally when I click on a block and go to the “Block Edit” mode, the block edit panel appears. When I am inside the block edit mode, I use the ‘Insert’ command to insert an external file as a linked block. The file is inserted as a block, but the previously open Block Edit panel is greyed out and the block which I was initially editing is not under edit mode anymore. Is there any reason for this? Thanks.