Since the very low voltage track circuit works by two wheels and an axle shunting from one rail to another, yes, sand can cause problems with shunting if a very short train (and here I mean a one or two-unit light engine) is stopped on heavily sanded rail. Please note the emphasis.
Since the change from pole line signal coding to electronic coding in the rail, there have been rare instances in which a short train of light equipment did not adequately shunt the signal system and road crossing warning devices. However, this is only a rare problem with trains consisting of very few axles. Most railroads have a rule that, unless a train or light engine exceeds a certain number of axles. That equipment must move only under positive protection of an absolute block established by the Train Dispatcher. Such a train may move on the main track within the absolute block, but must approach crossings protested by active warning devices (i.e., flashers, gates, etc., not just signs) prepared to stop until it is seen that the warning devices are functioning as intended. To avoid all of this, the usual remedy is for there to be sufficient axles in a train, even if the train has to tote a few extra cars.
Don't be fearful that this is something that can happen often or easily. It has happened on a tiny number of occasions, and warrants this abundance of caution because trains must be able to rely completely upon the block signal system.