Why is double -ll- used after a vowel digraph in "surveilled, surveilling"?

Why is the letter l double in the inflections of the verb surveil?

It's not in those of (as)sail or veil, and AmE has canceled but mostly cancellation. It's a counterintuitive spelling similar to those of control.

No wonder some people write it as surveille

Per Decapitated Soul's answer to When is "L" doubled?:

The -l- after a digraph or a diphthong does not usually get doubled.

Examples:

• Prevail -> prevailed not prevailled.
• Detail -> detailed not detailled.
• Mail -> mailed not mailled.

Why is surveilled, surveilling an exception?

Solution 1:

I'd guess it's because it is a back formation from surveillance, which is spelled with two Ls. As you noted, some people use the spelling surveille.

The noun surveillance remains much more common than any form of the verb:

(Google Ngram Viewer)