I have found that the following scenario behaves inconsistently (1.12.2 vanilla):
1) I power a command block with redstone torch => redstone dust => command block
2) The command block is this: /testfor @a[x=-241.5, y=64.0, z=-51.5, r=5] , repeat, unconditional, needs redstone
3) The command block powers a chain, one of which removes the redstone dust and thus disables the command block.
4) A world reset switch elsewhere restores the redstone dust.
5) My redstone wire, torch, and command blocks are all buried under the ground.
When I do this, it works as expected... sometimes. Other times, the testfor fails. In those scenarios, as soon as I dig in the ground to look at the redstone, it suddenly powers and the /testfor finds me.
Any ideas what I am missing here? Why does this work sometimes and not others? Is there something about me being able to actually see the redstone that causes some sort of game update to occur?