[throw0101c]

> It is breaking the spec […]

As per a sibling comment, it is not breaking the spec. The comment in the Pg code even cites the spec that says what to do (and is quoted in the post):

     * Generate UUID version 7 per RFC 9562, with the given timestamp.
     *
     * UUID version 7 consists of a Unix timestamp in milliseconds (48
     * bits) and 74 random bits, excluding the required version and
     * variant bits. To ensure monotonicity in scenarios of high-
     * frequency UUID generation, we employ the method "Replace
     * LeftmostRandom Bits with Increased Clock Precision (Method 3)",
     * described in the RFC. […]