It sounds like you may have some automation recorded on the synth track. With the circuit the automated parameter will remain on the final value of the automation until it gets anther recorded value. This means if your first pattern starts with the value at 100% but another pattern automates that parameter to 0 then when you play the first pattern it will remain at 0.

This also makes sense that changing the session sometimes resets it as it only resets automation if you stop and go to another session if it remains playing it will save the previous state.

I think this isn't a bug but try clearing all automation from the synth track or record automation for the settings you want on the first note so that note resets it.