What's a "perf" source?
If you run a btest from another device and not from the main router itself, as recommended, I think it'll all be fine.
I have corrected the word "pref source" in my post.
So my scenario is the following, I have a border router and I have an ASN. My provider gives me a /64 prefix not routed to the internet for p2p connection purposes to perform bgp, with bgp I can assign part of my /32 prefix to my edge router. But, if I want to start a speed test on my edge mikrotik, it will take from ip src the /64 that is not routed to the internet so performing the test is not possible. In ipv4 this is solved because in the Ip/route field you can select "pref. Source" to the default route, however in ipv6 this option does not exist.