Why is it so that if a Jew has a male child with a non-Jewish woman the child is not circumcised, even though, like Ishmail, the child is under Abrahamic (but not Mosaic) covenant, since the father is of "seed of Abraham"? (I assume Abrahamic covenant status follows the father, since it descends from Abraham to Ishmail and Isaac, and from Isaac to Jacob, from Jacob to all the other Jews and their offspring).
Also, such a child is not Jewish (not under the Mosaic covenant), but Genesis 15:18–21 promises all the descendants of Abraham specific land and to curse those, who curse them. This is different than for Bnei Noah where no such promises are made, so wouldn't that imply a sign of the covenant should be made for all the descendants of Isaac (Ishmail did not inherit the covenant:
וַיֹּאמֶר אֱלֹהִים, אֲבָל שָׂרָה אִשְׁתְּךָ יֹלֶדֶת לְךָ בֵּן, וְקָרָאתָ אֶת-שְׁמוֹ, יִצְחָק; וַהֲקִמֹתִי אֶת-בְּרִיתִי אִתּוֹ לִבְרִית עוֹלָם, לְזַרְעוֹ אַחֲרָיו
וּלְיִשְׁמָעֵאל, שְׁמַעְתִּיךָ--הִנֵּה בֵּרַכְתִּי אֹתוֹ וְהִפְרֵיתִי אֹתוֹ וְהִרְבֵּיתִי אֹתוֹ, בִּמְאֹד מְאֹד: שְׁנֵים-עָשָׂר נְשִׂיאִם יוֹלִיד, וּנְתַתִּיו לְגוֹי גָּדוֹל