It gets worse over time but it also eventually gets better, after the deleterious recessive alleles have been eliminated. Like in herd animals where a herd has only one breeding male per generation, so every generation is half-siblings.

The general rule is that a population with a fixed degree of inbreeding will have a corresponding number of deleterious alleles so that the selection pressure balances out; but when you change the degree of inbreeding, you get a spike in expressed mutations until things balance out again.