as of my understanding IPv4-embedded IPv6 containing private IPv4
address is supposed to be dropped either by the receiver or the
intermediate party (NAT64 translator)
That is only true of using the well-known prefix (64:ff9b::/96
):
3.1. Restrictions on the Use of the Well-Known Prefix
The Well-Known Prefix MUST NOT be used to represent non-global IPv4
addresses, such as those defined in [RFC1918] or listed in Section 3
of [RFC5735]. Address translators MUST NOT translate packets in which
an address is composed of the Well-Known Prefix and a non- global IPv4
address; they MUST drop these packets.
The well-known prefix is not the only prefix that can be used (see my highlights in the paragraph below the figure):
2.2. IPv4-Embedded IPv6 Address Format
IPv4-converted IPv6 addresses and IPv4-translatable IPv6 addresses
follow the same format, described here as the IPv4-embedded IPv6
address Format. IPv4-embedded IPv6 addresses are composed of a
variable-length prefix, the embedded IPv4 address, and a variable-
length suffix, as presented in the following diagram, in which PL
designates the prefix length:
+--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|PL| 0-------------32--40--48--56--64--72--80--88--96--104---------|
+--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|32| prefix |v4(32) | u | suffix |
+--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|40| prefix |v4(24) | u |(8)| suffix |
+--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|48| prefix |v4(16) | u | (16) | suffix |
+--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|56| prefix |(8)| u | v4(24) | suffix |
+--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|64| prefix | u | v4(32) | suffix |
+--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
|96| prefix | v4(32) |
+--+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 1
In these addresses, the prefix shall be either the "Well-Known Prefix"
or a "Network-Specific Prefix" unique to the organization deploying
the address translators. The prefixes can only have one of the
following lengths: 32, 40, 48, 56, 64, or 96. (The Well-Known Prefix
is 96 bits long, and can only be used in the last form of the table.)