I could not test it, as there is not a period of 28 nights available on your calender to book.
I saw the nightly price for shorter stays is indeed R$90,-.
Are you sure you did not set a monthly discount (or last minute, early bird discount or create ruleset with special discount) or have set specific price for specific month ?
As I (and@Charlotte495 also) mentioned: check all the possible discount settings. Guessing is not science. Try to make a 28 nights period available on the calender and simulate a booking, so you can test what is happening with monthly calculations and maybe get a clue what is wrong.
I think there are several places where things could be going wrong. There are multiple places to have discounts applied - different prices for weeday vs. weekends, seasonally, as a percentage for long term, etc. I'd go into every single setting on your listing and check. Also, if you look out at every day - like go on the calendar to the 102nd day - is the rate on that day still $90. Perhaps you've got "smart pricing" turned on in the future? That's my only ideas.