Converting a sprinkler system's 24V AC outputs to 3.3V DC logic inputs

the problem with your circuit is that the input can cross-talk via the bridge diodes and the capacitors.

in1-d1-c1-ground-d2-in2 and in1-d1-ground-c2-d2-in2 etc.

try this, it should give better isolation between inputs.

^{simulate this circuit – Schematic created using CircuitLab}

Optoisolators (AC input) would be an easy solution.

Or half-wave rectify the inputs (just a diode rather than a bridge and increase the capacitor values.. but it will be slow, which probably doesn't matter in this case).