I'm guessing that Poland doesn't want to end up in a direct conflict with Russia, so they're kind of gauging the situation and trying to figure out how far Russia is planning to go. I expect Russia wouldn't want to deal with occupying west Ukraine because that's where most of nationalist sentiment is. If Poland is dumb enough to make it their problem, I'm sure Russia will be perfectly fine with that. This would also severely undermine western narrative about trying to protect Ukraine.