Found that on upstream issue:
https://github.com/systemd/systemd/issues/25676

All needed is to fake content in signed zone, reported with unbound:

server:
  local-zone: example.org typetransparent
  local-data: "example.org. 3600 IN A 127.0.0.1"



Reproducible: Always

Steps to Reproduce:
1. Enable DNSSEC=yes
2. Run local unbound, configure fake local-data
3. Set DNS=127.0.0.1
4. resolvectl query -t example.org
Actual Results:  
[root@rawhide ~]# resolvectl query -t a example.org
example.org IN A 127.0.0.1

-- Information acquired via protocol DNS in 8.5ms.
-- Data is authenticated: no; Data was acquired via local or encrypted transport: no
-- Data from: network
[root@rawhide ~]# resolvectl query -t aaaa example.org
example.org IN AAAA 2606:2800:220:1:248:1893:25c8:1946

-- Information acquired via protocol DNS in 10.2ms.
-- Data is authenticated: yes; Data was acquired via local or encrypted transport: no
-- Data from: network

Expected Results:  
Similar to when signature is present, -t a should be reported as invalid, only -t aaaa successful.

Marking it with high severity, because it undermines purpose of whole DNSSEC presence.