This issue happens to me too with a regular piston.
I did a simple redstone clock with 4 repeaters and it worked as intended, but when i added another one the piston no longer retreats, even if i broke the redstone circuit, i does after a block update next to it. (the repeaters where at 4 ticks, the observer was just to start the clock)
[media]
Sorry about that.
When you use the Shield to block an arrow (also tridents) the animation when hit makes the shield to look "in rest" position but still able to block with it. (Without releasing the right-click at any time).