OP - I think I know what your problem is because I had the same problem 2months ago. In my case, the toilet also wouldn't hold water after finishing the flush process for more than a few minutes helping provide a clue.
The way the wire mechanism in the back works is to slide in a slot on the plastic valve due to resistance between the plastic toilet gate and the rubber gasket around it. If the screws/bolts (4) holding the top of the toilet assembly down tightly, and thereby compressing the gasket around the gate, loosen up the gate/rubber friction isn't high enough to slide the wire in it's channel and it just opens the gate right away.
In my case I completely took the toilet off and took things apart trying to understand how everything worked before discovering the root issue. After the fact I understood I could just remove the shroud around the base to tighten the bolts/nuts holding the top of the toilet to the bottom and compressing the rubber gasket. After tightening these the bowl held water again, and the fill without flush when initially depressing the footpedal worked again too.
Not a difficult fix once you know what the issue is, probably take you 30min including fumbling around and double-checking everything.
Good luck,
Ron
Ron & Pat
2022 F350 Lariat CCSB SRW Diesel
2019 VanLeigh Vilano 320 GK