This is working as intended. The luxury tax is calculated based upon the amount of luxury tax owed - luxury tax you've paid already divided by the number of days left in the season. Some examples:
A team is 10 million over the salary cap on day 1. They would pay $250,000 (10 million/40) a day in luxury tax.
On day 10, this team goes to 20 million over the salary cap. They have paid 2.5 million in luxury tax thus far. They now owe 17.5 million in luxury tax and would pay $583,333 a day in luxury tax (17.5 million/30).
On day 35, this team goes to 30 million over the salary cap. They have paid 17.1 million (approx) in luxury tax thus far. They now owe 12.9 million in luxury tax and would pay 2.58 million a day in luxury tax (12.9 million/5).
A team is 10 million over the salary cap on day 1. They would pay $250,000 (10 million/40) a day in luxury tax.
On day 10, this team goes to 20 million over the salary cap. They have paid 2.5 million in luxury tax thus far. They now owe 17.5 million in luxury tax and would pay $583,333 a day in luxury tax (17.5 million/30).
On day 35, this team goes to 30 million over the salary cap. They have paid 17.1 million (approx) in luxury tax thus far. They now owe 12.9 million in luxury tax and would pay 2.58 million a day in luxury tax (12.9 million/5).