This cannot be explained without going into a bit of history and a long ramble about standardisation.

The discussion is similar to the one in English whether 's' spelling is correct or 'z' spelling in words like standardisation, organisation, etc.

Similary there are two schools of thought in Hindi regarding the nukta question, whether to put nukta in words like सफाई, जमीन, कफन, नकाब, etc. or not

Grammarians like Kishoridas Vajpeyi (Author of Hindi Shabdanushasan, an authoritative grammar of Hindi) allows for the nukta in only two letters, ड and ढ.

His argument is that all other sounds such as ज़, क़, फ़, ग़, etc., are pronounced by educated Hindi speakers without the nukta-induced sounds. And therefore, there is no point in attaching these to these letters.

A complicating factor is that Hindi and Urdu are one and the same languge written in two different scripts - devnagari and Urdu. The urdu script incoroprates a number of sounds of Arabic, Turkish and Persian which do not exist in the devnagari script. To represent these sounds, the nukta or dot is put below related sounds of the devanagari script.

But this is merely pedantic. Very few Indians, including Urdu speakers, now know how to correctly pronounce these Arabic, Turkish and Persian sounds because facility for teaching these classical languages in India has disappeared more than two hundred years ago with the establishment of the British Raj which displaced Persian with English as the court language and promoted mass education in English and regional languages.

Because of the huge market available in Hindi, many Urdu writers now publish their works in Devnagari. When Urdu writing is published in Devnagari, they put the nukta on words of Arabic, Turkish and Persian origin.

But many of these words have over the years adapted to the phonetics of devnagari and are commonly pronounced as they are written in devnagiri, that is without the nukta. Examples are words like जमीन, फूल, फन, नकाब, कब्र, सफाई, साफा, etc. These should be written without the nukta.

In my opinion the nukta should not be used in Hindi at all except in ड and ढ.

If Urdu text is being quoted in a Hindi treatise, or Urdu text is being written in devnagari script, only then should the nukta be used.For example, if you are quoting Galib in your essay, then the quoted text could have nukta, but not the main body of the essay which is being written in Hindi.

In other words, in all day-to-day Hindi translations and writings, the nukta should not be used in words like सफाई etc.

In Hindi the correct spelling is सफ़ाई, and सफाई is wrong as can be seen from the sentences quoted from internet magazines also. Both are two different characters with different pronunciations and sometimes the words formed with these alphabets have entirely different meaning like फन (the hood of a snake) and फ़न (skill or art).