Sforno on Numbers 1:20/ספורנו על במדבר א׳:כ׳

1בכור ישראל. He had not lost his status in the books kept in heaven as he had repented his sin as testified in Genesis 35,22-23 commencing with the otherwise unnecessary words ויהיו בני יעקב שנים עשר, “that Yaakov’s sons remained twelve.” This was in contrast to Reuven’s birthright privileges here on earth having been revoked in favour of Joseph.