My Bunn coffeemaker has started overflowing at the funnel
It's possible that the soft water is helping the coffee &quot;bloom&quot; (and eventually overflow past the top of the basket) more than when you were using harder/more mineralized water. All else held equal, there's a few things to try to solve the concern, as one or a combination of the following may work.
Try:

...reducing the amount of water you place in the tank, and expect smaller/less servings.
...putting in less coffee in the basket, balancing the risk of getting &quot;watered-down&quot; coffee (see #1 above).
...using/buying coarse ground coffee instead of the fine powder (for the reasons below)

Now if you grind the beans yourself, also try:

...using a burr grinder (instead of a blade-type) for more consistent coffee size, as the fines from using a blade grinder may be clogging your mesh/paper filter (not the nozzle!)
...grinding the coffee a little bit coarser to give more space for the water to run through.

Hope this helps.