Set Row Playback Mode to Forward    ⌃]

From the new Step Sequencer in Logic Pro X — 10.5.

At some point I will need to go way back and re-visit my days of sequencing using Opcode’s “Vision”.

Edit Step Sequencer pattern, row, and step settings in Logic Pro — Apple Support

To change the playback mode for the row: Click one of the Playback Mode buttons. The choices are, from left to right:

• Forward: Steps play from left to right.
• Backward: Steps play from right to left.
• Ping-Pong: Playback alternates between left to right and right to left.
• Random: Steps play in an undetermined order.