The issue discussed here seems to be not an unbound issue. That's why changing unbound config doesn't help. The key statement from the unbound GitHub open issue related to this one is:
dnsmasq queries Unbound over TCP (not sure why, probably because dnsmasq was queried over TCP as well),
Link: failed to send TCP(read_write) packet (Connection prematurely closed by remote server) · Issue #1237 · NLnetLabs/unbound · GitHub
Is there a reason why dnsmasq started to query unbound over TCP more often in pi-hole v6? This warning/error didn't occur in v5.