Perhaps we should remember the English spelling rule: "For every rule there is an exception"!
Indeed. To quote one of the better-known 'rules' -
'i' before 'e' except after 'c', yet we find words like
seize and
protein ignoring this rule.
However, these two words would still be pronounced in the same way if they were to be spelled
sieze and
protien.
A foreign learner would have no problems pronouncing either of these forms.
I'm not sure that that is true with
nonplused.
A foreign learner might well have to ask if the second syllable was pronounced to rhyme with 'bust' or 'boozed'!
To quote Michael Swan [in 'Practical English Usage'] :
The reason for doubling is to show that a vowel is pronounced short. This is because, in the middle of a word, a stressed vowel letter before one consonant is usually pronounced long or as a diphthong He illustrates this with examples
'hoping /hopping, later/latter and diner /dinner'.I realise that Professor Swan uses the word 'usually', which implies that there may be exceptions to the 'rule', however a comparison of
nonplussed / nonplused on the Google NPlan Viewer for all varieties of English can leave little doubt that, if not wrong, the 'one-s version' is most definitely the less common of the two forms.