If your Schedule K-1 is showing unallowed prior year losses and current year unallowed losses in box 2, but prior year unallowed losses are not showing on Form 8582, Passive Activity Loss Limitations, you have most likely indicated an amount in box 1 as nonpassive and an amount in box 2 as passive on the same K-1.
Per the instructions for Form 8582, page 9:
Column (c). Enter the unallowed losses for the prior years for each activity. You find these amounts on Worksheet 5, column (c), of your 2017 Form 8582. Enter the total of column (c) from your 2018 Worksheet 3 on line 3c of Form 8582.
In order to get this to work in the TaxAct program, follow the instructions below:
Schedule K-1 for Form 1065 (Partnerships)
Schedule K-1 for Form 1120S (S Corporation)
You should now see the amount transferring to Form 8582 as expected.