I want to reach my IPv6 home network from a location that has only IPv4 connection. From what I've read this can work with 6to4 tunnel brokers.
Following a 6to4 tutorial using tunnelbroker.net, after adding all relevant information to my hex S the normal IPv6 internet connection is lost.
Normally, I have a public IPv6, but as soon as I activate my 6to4 interface, the public IP is lost.
Is this expected behaviour?

6to4 is a service to tunnel IPv6 over an IPv4 network and cannot be used to expose your IPv6 device over IPv4.
You will need some other (VPN) service for that, providing a public reachable IPv4 address.

Also, v6 routeros does not support ipv6 route rules, so multihoming IPv6 is not really going to work.

You can however use the tunnelbroker service on the (IPv4 only) client side to be able to reach IPv6 device from the IPv4 connection.