One of the Baroness’ skills in The Huntress skill tree (probably Silver Lining, Magic Bullet, or I Never Miss) causes sniper rifle critical hits to not be registered as such for purposes of Badass Ranks, meaning that Melon Splitter and Critical Reception stop progressing.
Magic Bullet seems to be the culprit. I tested it extensively and came to this conclusion. I brought it to @Jeffybug’s attention. He was going to pass it on to the team. I just removed it, completed my crit challenges that way.