Given the following example, the second assignment to 'a' never occurs. I have also witnessed the same issue when the timescale is as precise as "1 s / 1 ns".
`timescale 1 s / 1 fs
a = 1'b0;
a = 1'b1;
$dumpfile( "timescale1.18.vcd" );
$dumpvars( 0, main );