Some interesting reanalyses going on with this root, you also get מוּל and נָמַל. Wonder if some kind of half-hearted taboo avoidance caused speakers to get more creative with it, or to avoid the word frequently enough that learners didn't get the full paradigm.
Brown-Driver-Briggs BotEntry: [מָלַל] vb. circumcise Qal Jos 5:2. Niph. Gn 17:11 Hithpo. be cut off: ψ 58:8.