I have made a test between RB3011/RouterOS 6.49.5 and a CHR/RouterOS 7.2rc7
When I create eoip (ipv4) interfaces on both sides and assign private IPv4-Adresses to both ends, I can ping them as expected.
When I assign IPv6 (fd::1/126 - fd::2/126) to both ends, I can not ping the other side.
Doing the same with eoipv6 works as expected, IPv4 and v6 Packets will use the tunnel, with eoip (IPv4) only IPv4-Packets goes through the tunnel