Apart from the normal format for IPv6, it is also possible to format an IPv6 with an embedded IPv4 instead of the last two blocks. This is described in the RFC 4291 section 2.2.3, for example:
::FFFF:129.144.52.38
The format is used for systems working with both IPv6 and IPv4 addresses. However, in the RFC it is not made clear if this format can also be used to display normal IPv6 addresses or not.
It is important that anonymizers recignize this format as well, since privacy regulations also require it to be anonymized. Therefore, a solution should be found that can do that.
anonymizing IPv6 with embedded IPv4