kzonecheck fails on case-sensitivity of owner names in NSEC records

kzonecheck fails on case-sensitivity of owner names in NSEC records.

Tested versions: 2.9.6/2.9.7/3.0.1

The following error message appears "incoherent NSEC chain".

Steps to reproduce

create key

dnssec-keygen -a ECDSAP256SHA256 -f KSK foo.
Generating key pair.
Kfoo.+013+17470

create zone

Note, the capital letters "NEW".

$TTL 86400 ; (1 day)
$ORIGIN foo.
@ IN SOA ns1.foo. hostmaster.foo. (
                1          ; serial 
                14400      ; refresh (4 hours)
                1800       ; retry   (30 minutes)
                1209600    ; expire  (2 weeks)
                3600       ; minimum (1 hour)
                )
 
                IN   NS    ns1.example.com.
                IN   NS    ns2.example.com.

                IN   A     203.0.113.10
                IN   AAAA  2001:DB8:BEEF:113::10
www-NEW         IN   CNAME example.com.

$INCLUDE Kfoo.+013+17470.key

sign zone

dnssec-signzone -o foo. -O full -P -t -x -z foo.zone Kfoo.+013+17470.private 
foo.zone.signed
Signatures generated:                        8
Signatures retained:                         0
Signatures dropped:                          0
Signatures successfully verified:            0
Signatures unsuccessfully verified:          0
Signing time in seconds:                 0.001
Signatures per second:                5021.971
Runtime in seconds:                      0.005

verify zone with kzonecheck

Version used

kzonecheck -V
kzonecheck (Knot DNS), version 3.0.1

kzonecheck -o foo. foo.zone.signed 
[foo.] incoherent NSEC chain
[www-new.foo.] incoherent NSEC chain

Error summary:
   2	incoherent NSEC chain

As a reference, signed zone file which contains www-NEW.foo. in capital letters

cat foo.zone.signed 
; File written on Tue Nov  3 17:11:16 2020
; dnssec_signzone version 9.16.8
foo.					      86400 IN SOA	ns1.foo. hostmaster.foo. 1 14400 1800 1209600 3600
foo.					      86400 IN RRSIG	SOA 13 1 86400 20201203151116 20201103151116 17470 foo. 0cjFMPyfCO9V+hjuLm5Ank5CTOXXR3Wpbon2tUDGYHolGfwclmXUPRW2 yD4ybRo9/87VYTR5XBFTXc6OdLJPCg==
; resign=20201203151116
foo.					      86400 IN NS	ns1.example.com.
foo.					      86400 IN NS	ns2.example.com.
foo.					      86400 IN RRSIG	NS 13 1 86400 20201203151116 20201103151116 17470 foo. P5Vettob2iwqApHhZ5fdfcez7k2lPtBKjyTZEHm0QOziEYfCuXptrKpZ 7Q3gitOpDZQcXlXQ6BSFn3b/sLAPIw==
; resign=20201203151116
foo.					      86400 IN A	203.0.113.10
foo.					      86400 IN RRSIG	A 13 1 86400 20201203151116 20201103151116 17470 foo. cXno7F2TDqKbJLY07wG8nFECIAxh3DpoxSVVpFGgbjyV4QXq5YBmL2xb mmCs55wvlq58dkb1tbaWncoCs03jGA==
; resign=20201203151116
foo.					      86400 IN AAAA	2001:db8:beef:113::10
foo.					      86400 IN RRSIG	AAAA 13 1 86400 20201203151116 20201103151116 17470 foo. hzE6R1NnqWu9fUreeZj6QODjSl1ausDz3tZqnRaRp/KTCW8+HJxJJy4q geN6k7A0TOM7izvyd5dZAY6IZ+9Fbg==
; resign=20201203151116
foo.					      3600 IN NSEC	www-NEW.foo. A NS SOA AAAA RRSIG NSEC DNSKEY
foo.					      3600 IN RRSIG	NSEC 13 1 3600 20201203151116 20201103151116 17470 foo. es2cO3/dIO8/SbD5YyoYwpZff2O1LS6S0L6XyomKl3aQHIRyXEQskxX1 21KY3SmDEs8ifb4xxB9WxniZ+CsOgw==
; resign=20201203151116
foo.					      86400 IN DNSKEY	257 3 13 wWp1TLQRFkBQyzKVxL7cKoKdph0PwUBrgYguZt3LmxeOuLMEVvnU1UrB vcFWbQvDZRR3l7AvvyMqkbz31CaZNA==
foo.					      86400 IN RRSIG	DNSKEY 13 1 86400 20201203151116 20201103151116 17470 foo. cjl9vK3whVGaWpmq3pKV8cDjLJPwrm4cbJEO+93f0DLUZXBsDCEmetj2 wsSo90tCQG+rmsAo3PR0iTItgjmZ5w==
; resign=20201203151116
www-NEW.foo.				      86400 IN CNAME	example.com.
www-NEW.foo.				      86400 IN RRSIG	CNAME 13 2 86400 20201203151116 20201103151116 17470 foo. OyAEfGB5h5xS2MroKW8qpIhvJSbpP6GCT1mZipoSQDu3WRMNw1P7sVT5 q75NqTgiB/LNwyd9BWQ2gC/a9lWk/Q==
; resign=20201203151116
www-NEW.foo.				      3600 IN NSEC	foo. CNAME RRSIG NSEC
www-NEW.foo.				      3600 IN RRSIG	NSEC 13 2 3600 20201203151116 20201103151116 17470 foo. T//2Y56YYvWCXE6oeR0RrBEnsjBVE1nKkc0ogWe15GlbeCGdVsEKymn2 mTb1+I3Vvhv/e57nCKoPdgxcB8IrNA==
; resign=20201203151116

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