When I blew up the tube to test my patch
Maybe this is just superstition, but I try to avoid "testing" the patch like this (blowing up the tube outside the tire), under the belief that such "testing" is pretty likely to actually
cause the patch to fail.
1) You can see the divot in your tube where the patch is keeping it from expanding as much as it wants to. That means the tube is trying to stretch under an unstretchable patch, which must put undue strain on the glue.
2) The air pressure inside the tube wants to squeeze out of the hole, and the only thing stopping it is the glue holding the patch in place. When the tube is safely inside the tire, it has both the glue and the pressure from the surrounding tire to hold it on.
So my strategy is to blow it up as big as necessary to
find the leak, but once it's patched, don't blow it up again until its inside the tire. I sorta just feel like a patch isn't truly "seated" until it's spent some quality time getting smashed against the inside of a tire.