Yea, that a systemctl optimization for Nano that prepends +{line-numer} to the file name to get the editor to start at the specified line. The Kate-ism for this is to postfix the line number to the filename with a colon (i.e. {filename}:{line-number}), but my favorite terminal text editor - mcedit supports both the + syntax and the : syntax, so maybe Kate can be convinced to also support both.
I think its a behavior of -n, but I’m not sure.
This actually the expected behavior because code is not a command line editor and detaches from the terminal immediately. Kate does the same, which is why it also isn’t a good idea as a replacement for a command line editor - at least not the way I said it should work (my bad, sorry). Try to set EDITOR="kate -b" instead - to get Kate to behave like a proper command line editor (it will still open the weird “+4” tab, though).