Thanks for the hint.  The problem is fixed.

Server was setting:

if (SSL_CTX_set1_groups_list(ctx, "X25519:P-256") == 0) {

The call succeeds.

But the old TLS 1.2 code was setting:

            int nid = NID_X9_62_prime256v1;
            EC_KEY* ecdh = EC_KEY_new_by_curve_name(nid);
            if (ecdh == NULL)
            SSL_CTX_set_tmp_ecdh(ctx, ecdh);

After disabling this, the server responds with the right group - X25519.

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