I am running RouterOS v6.47.9 on a hex s.
My internet connection is via PPPOE.

My PCs on the LAN get DHCP entries from the Mikrotik router. DNS servers are specified in the DHCP server networks section. In the PPOE section the Use Peer DNS is unchecked.
However, PCs on the LAN are getting an additional IPv6 DNS entry as well as a search domain entry that I haven't specified and I can't find in the router settings. I don't know where these entries are coming from.

This has only started to happen since my internet connection moved to a new provider and over PPPOE. Prior to this DHCP and DNS entries behaved as expected.

Can anyone help with where the unspecified DNS entry is coming from? How is it getting in the DHCP settings to my PCs (Macs)?

How have you configured your IPv6 WAN and LAN settings? The PPPoE client 'Use peer DNS' setting only applies to IPv4, similarly the DHCP(v4) server network settings.

I don’t think I’ve done any configuration as I’ve not seen anywhere to do it. Where do I configure ipv6 in webfig?

what would this "additional IPv6 DNS entry" be?

routerboard do not have any IPv6 settings until on system / packages the IPv6 is disabled
on default IPv6 on 6.x version is disabled
Probably RouterOS and his configuration do not have any to do about that.

Check also if your OS do not establish automatically a teredo tunnel

Thanks. I checked the Mac settings and changed the IPv6 configuration from Automatic. And the IPv6 DNS entry is gone. Perhaps the behaviour changed with the upgrade to Mac Monterey.

Is there any way to block this activity on the router or must I configure each PC?

Each PC,
but if RouterOS have IPv6 disabled, how can be influenced the internal IPv6 network?