The dispensers are filled with water buckets, and fed by redstone torches as we can see in the screenshot. When the redstone torch is turned off or turned on, the dispenser doesn't change from is previous state :
The torch is on, and water is dispensed. The torch turn off, the water doesn't come back in it.
MC-91635
Dispenser does not actualize his state
Resolved
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You need to power the dispenser again to get it to collect the water again, that's a normal feature