Solution:

In the 03: [Machine: Paper Feed/ Output] group of the [Adjustment Settings for Operators] menu, set 0301: [Correct Output Paper Curl: 1 Sided: Face Up], 0302: [Correct Output Paper Curl: 1 Sided: Face Down], or 0303: [Correct Output Paper Curl: 2 Sided] to "П Curl Correction Level".

Alternatively, in [Advanced Settings] for the custom paper, select 1311: [Correct Paper Curl], and then specify "П Curl Correction Level".

If "Off" is selected, change the value to "Small". If "Small" is selected, change the value to "Large".

If you cannot remove back curls, print with the printed side face up for single-sided printing.