Closing water supply valve does not shutoff water supply to toilet
Does this mean that the supply valve has lost its grip...?&quot;

YES. Your valve has failed and it is very common for &quot;bib&quot; type stop-valves to fail this way. It will need to be replaced which requires shutting off upstream (maybe even all the way upstream, as in the whole house). For a couple bucks more you can get a ball-valve type, which should be more reliable for you. I can sometimes get failed bib-style stop valves to shut off by really cranking them tightly (which destroys them forever... but it was already jacked up, right?)