I would never have a Push-Pull at logic 0....
Here's what I'm thinking.
1. all columns are sitting as open drain at logic 0
2. a row (which is open-drain at logic 1) goes low (aka button is pressed)
3. configure first column as push-pull
4. set first column at logic 1
5. check if row went back to a 1
6. configure first column back to open-drain
7. set first column to logic 0

so there shouldn't ever be a Push pull at 0 to cuase problems with a push-pull at 1.
Right?