Though correct transliteration of lyp-sync is липсинК, липсинГ is often used instead, because the word is not listed in dictionaries, and as such, Russian speakers use their language feeling to decide which writing form suites best in this case. Note that most Russian speakers do not know English, so they do not use English rules here.
So, from a perspective of Russian-speaker, a word that sounds like [липсинк] can be written in two ways: липсинк and липсинг (both writings are pronounced the same way - [липсинк], this process is known as devoicing). Russian speaker knows that many borrowed words (they may or may not be aware of English origin of those words) have 'инг' at the end: фактчекинг, лизинг, конвертинг, спарринг, допинг, шопинг, тюнинг, кастинг, тренинг and so on and so forth. Since [липсинк] obviously goes to borrowed category, it seems that 'инг' ending is 'more correct' than 'инк' ending. This version is also backed by the fact that writing 'инк' in this position instead of 'инг' is a common spelling error (since both endings are pronounced the same way).
And in fact 'инк' ending is not common to other Russian words as well. Almost all nouns that have such ending are limited by: цинк, сцинк (lizard), инк (one of Inca people). Other words are proper and family names like Метерлинк, Хиддинк, Пинк Флойд. Non-listed in dictionaries, but known are линк (пруфлинк, гиперлинк), дринк (софт-дринк, лонг-дринк). So we have here only 10 nouns (plus 20 family names), while there are over 400 words ending with -инг.
Such transformation, based on common native speakers impression what is 'correct' and what is 'incorrect' resulted in modification of borrowed or existing Russian words with spelling similar to common spelling errors:
- Initially Фляшка - later replaced with фляжка (borrowed from Polish flasza, looks like шк/жк misspelling)
- Пошта - почта (borrowed from Polish росztа, looks like шт/чт misspelling)
- Машта - мачта (borrowed from Dutch mast, looks like шт/чт misspelling)
Though липсинг may look more correct for a Russian eye (non-aware of English grammar and phonetics), the fact that most dictionaries do not contain this word, and existence of дринк and линк leaves a chance that consistent usage of липсинк will overcome incorrect transliteration.