In past years, Turbotax calculated the maximum 401(k) contribution to equal the net income. In 2018, it calculated only 25% of net income. Has something changed?

This is a small side business with net income typically less than $15,000.  In 2016, and 2017 the max contribution was calculated by Turbotax to basically zero out my profits ($19k and $7K respectively).  But for 2018, the max is calculated as exactly 25% of net income.

Has the new tax law changed the deductible amount?  Or am I doing something wrong?