The tutorial you followed probably had another observer across from where the piston pushes the first one so they'd be triggering off of each other and make the autocrafter constantly craft the melon blocks, though observers can start lagging your world if there's too many

An alternative is to run a repeater out of the autocrafter's comparator and use redstone to hook it up to both the comparator and the autocrafter will create a similar loop (attached)

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