Hi, The wiki doesn't say that you should always prefix the value of addr:flats with a semicolon, only when the first unit doesn't have flat number which i doubt is the case here.

"A leading semicolon can be used when a flat number is implicit. For example a house at '10 Some Street' converted into 3 flats with postal addresses of '10 Some Street', 'Flat 1, 10 Some Street' and 'Flat 2, 10 Some Street' can be tagged addr:flats=;1-2" 