Some drivers have a setting that disables them or at least lowers the current after a certain amount of time so they stay cooler. I don’t know whether it’s your drivers or the bee, but that would be my guess.
Details: A value of 255 will keep the motors enabled at all times (preferred for most projects). Any value between 0-254 will cause the disable pin to activate this many milliseconds after the last step. Note: Motors can be manually disabled at any time with the $MD command.