Kinda a well known issue of either the octocoupler or Triac for channel 31 and 32.
Try to isolate the issue to either the octocoupler (easy quick replacement, but more expense) or the Triacs (harder replacement, but less expensive).
First, while grounded (strapped in), swap the small 6 pin chip (the octocoupler) from channel 30 with the one on 31.
Power back on and test channel 30 and 31. We know that channel 30s octocoupler works so we are seeing if the problem moves to channel 30 after the swap. If the problem still persists with channel 31, then most likely your triac is bad. If channel 30 now is constantly on, then you need to replace your octocoupler (you will have to buy replacements)
If you suspect it is the triac, then you will have to buy new triac chips and you will have to unscrew the bad triac from the heat sink. You will have to clip off the legs (three of them) from your solder job and pull the remaining bits of leg by desoldering them. Make sure to clean up all the solder using a desoldering iron or using desolder wick. solder the new triac back into place for the bad you just desoldered. Test to make sure this does the trick.
Good luck.
Kurt