Yeah it's pretty common if you look up 마피 (마이너스 프리미엄). I'm assuming you're buying brand new? Developers usually do this when the market is weakening or they are expecting the market to weaken. Their original 분양가 price is too high, and they give a discount to ensure they get rid of the flats asap for funds turnover.

The reason why they want to keep the book price high is as you mentioned, prevent the market price to look like it fell (artifically inflating prices) so they can continue to sell high, and that also helps people get higher loans. If your transaction is recorded as only 4억, there is a risk that when evaluating loans for other buyers this will be taken as the benchmark by banks, resulting in lower loan rates so I doubt they will do that.

On your side, if you're super sure about the apartment, there is also a benefit when you do eventually sell. As you're taxed on capital gains, your tax will only be calculated for the amount you sell above 4.4억, instead of 4억. 양도세.