I do not speak for TASO on this issue, and TASO execs might have a different tack for this, but this would not, IMHO, warrant a UIL Incident Report. I would alert my chapter execs, and suggest that a memo be sent to the school to note the equipment failure, with our best wishes for having the equipment repaired before the next game at that location, and that a memo be sent to the rest of the chapter to notify them of the failure, and to review procedures and be prepared to keep the game/play clock timing on the field, particularly for those that may have future assignments at that location.
FYI, if this was an NCAA contest, yes, I would note the clock failure in my game report. You can be nearly 100% certain that the problem will be corrected before the next home game, one way or another.
But, we must always be prepared to keep clocks on the field.