Unfortunately this is a Google server fault.
While it accepts the new label/folder name, and syncs that back to eM Client, it neglects to say there are messages for the new folder. So there is nothing to display.
However, you can right-click on the All Mail folder in eM Client and choose Properties > Repair > Repair, which will force a full resync with the server.
This was also mentioned by @cyberzork in a another topic here: eM Client MacOS Gmail Label issue - labels empty after renaming parent label - #6 by cyberzork