Thank you Dave,Srini and Ben.
To summarize
(1) 1800, 16.12.22 terms concurrent assertion ‘hard_zero’ as degenerate sequence .
(2) Tools seem to evaluate the (0) at run-time due to which the fail action block executes on each clocking event. Hence instead of compilation error ( since it’s degenerate ) we observe run-time error
EDIT: As discussed in edalink, the error due to degenerate sequence could be at compilation / run-time