You are either loosing the speed signal of one of the front wheels or you are loosing the ABS system in general.
The vehicle speed signal is taken from one of the front wheels via the ABS wheel speed sensor. The raw AC signal is processed by the ABS controller and passed to the various users of vehicle speed (the speedo is one) as a pulsed DC signal. This is all easily verifiable with a scan tool using actual values.
Stated differently, the ABS will recognise a bad sensor and close down ABS function (and turn on light). The vehicle speed signal should be checked on a wiring diagram as I am not sure what happens to that signal if the ABS control unit is turned off.
Bosch Master, ASE Master, L1
33 years MB technician