I have all my low-voltage (3.3 V and 5 V) devices grounded together going into the low side of the 24-to-5-V stepdown converter, then the high side is grounded back to the battery.
Here's the verbatim reddit post:
I'm using one of these to power two motors at 24 V (manual here). At first, I was sending 5 volts of power to the logic pins and using an Arduino Uno to provide 5-V logic signals. Everything worked fine. Then, with the same 5 V logic power, I used an Arduino Nano to provide 3.3-V logic signals, which seems to have messed up the board. No motors will turn with the 3.3 V logic, and when I used logic level converters to step the logic up to 5 V, the motors will turn but now seems to be controlled by a random selection of Arduino pins (see this post if you're curious on that).
Would that difference in voltage between logic power and logic signals mess up the driver board?