Communication with Smoothieware fails at slower movement speed

I am using an AtomStack L40 laser module with an OX CNC 1000x1500mm belt-driven setup. The controller is an MKS SBase board running Smoothieware, using SteppersOnline external drivers.
First of all, I am getting good results both with raster (image) burns and lines (straight line, rectangle, circle). The problem arises when I am moving too slowly. Usually it is 20mm/s where things fail.

What happens is this: I get part of the way through a job, and the laser shuts off (I am using Smoothie in GRBL mode where G1 with S1000 will run the laser in PWM mode). The bar remains green, and I can try stopping the job but have to actually disconnect and reconnect (I am communicating via USB from a Windows 10 laptop). If I did not stop I don’t even need to clear the alarm.

It seems almost as if there is a timeout in communicating with the serial port, and LightBurn stops receiving on the port. I cannot guarantee there isn’t something odd happening with the Smoothie firmware, but I have verified that saving the gcode to a file and playing it from the Smoothie board’s SD card works. However, when I issue the “play sd/filename.gc” command from LightBurn, it will stop responding after the first movement once the play is completed.

I couldn’t find where to set communication parameters for the serial port with Smoothie selected. I have “show all” turned on in the console. I also did not see a way to reset the device connection (other than pulling out the USB plug and reconnecting it).

A workaround is to find the right speed so that I don’t have any single pass that takes too long.

To reproduce the problem, I created a job consisting of a single line 700mm long cutting at 90% power moving on only one axis (X) at 20mm/s. I set it to run for 8 passes (I’m trying to cut through 5mm plywood). Sometimes it will complete 2 passes (I’m running bidirectionally) and sometimes 3 passes, but at 20mm/s it will always stall. Waiting does not accomplish anything.

If I change the speed to 35mm/s, everything works as expected.

Thoughts?