As far as I know all じる verbs are supposed to be 一段.
I found an interesting set of rules online:
Does the verb end in る?
No → 五段
Yes → Does it have an e or i sound before the る?
No → 五段
Yes → Is the e or i sound part of the okurigana of the verb?
Yes → 一段
No → Could be both.
And someone posted the following counter examples to rule 3:
So I looked it up and found a few explanations for why these aren't valid examples.
脂ぎる seems like it's just 脂 + きる (unsure which kanji).
捩じる is 捩る or maybe some funky 二段/四段/s-irregular stuff. (Still wondering though, because じる was made for 一段.)
抓める is 抓る.
I found that 混じる can also be 混る, but that spelling is listed in no dictionary.
So am I just ass-pulling stuff, or is the rule wrong or is まじる really special?