Adding a new record works:
example.xn--gja.com. 300 IN A 10.0.0.1

nsupdate commands queued:
server localhost
zone xn--gja.com
update add    example.xn--gja.com. 300 IN A 10.0.0.1
send
answer

next edit contains this:

example.ǁ.com.            300   IN A     10.0.0.1

deleting this results in:

nsupdate commands queued:
server localhost
zone xn--gja.com
update delete example.ǁ.com. IN A 10.0.0.1
send
answer

which obviously fails:

update failed: NOTZONE

tested using version 1.2 on debian.

can't even workaround for editing by replacing the name with the punycode name in the temp zonefile, as it'll think i've removed the entries with non-punycode.