/ac "Cure" <f>

This is the only macro I use for a WHM for curing Tanks. And it works for me flawlessly.

Even when manually targeting to heal, my heals still go through to the target I'm trying to heal, no problem there.

The only way I've got this problem to occur is using a macro that failed to execute, such as:
/ac "Cure" <tt>
/ac "Cure" <t>

or using no macro but attempting to heal a hostile target.

/ac "Cure" <f>
/ac "Cure" <mo>

These macros will not execute at all if these conditions aren't met.

If you have no target, just casting a Cure(No macros) spell will not execute. However, Physick, on the other hand does execute even if you have no target.