Problem in Rule : EN_A_VS_AN

I run the following sentence on LT:
designed an user friendly and effective information system for the political office after recognising need for better organisation of the increasing data from the constituency of the minister.

And it changes the ‘an’ before user friendly to ‘a’. Why should that be the case?

While the general rule is to use “an” for words starting with a vowel, one should look at wether it phonetically starts with a vowel, as the purpose is to separate the indefinite article from the following noun.
“user” starts with a phonetic consonant, the epsilon. (compare “user” vs “youser”, most would pronounce both (nearly) the same way)