The 'constant_placeholder' definition incorrectly matches the '%s' in a literal string '%%s' and highlights it; it should match the '%%'.
Fix: in the constant_placeholder definition, change:
match = '%(([a-zA-Z_]+))?(0|-|+)?([0-9.]*)?[diuoxXfeEgGsrc]';
to:
match = '%(([a-zA-Z_]+))?(0|-|+)?([0-9.]*)?[diuoxXfeEgGsrc%]';