This is very much a bug in nvidia-graphics-drivers-340. One can debate if software-properties should or shouldn't purge (though traditionally, the argument was that silent frontends shouldn't purge because the user hasn't explicitly asked for that), but if a removed package's unpurged conffiles cause the system to misbehave, that package needs to be rethought to avoid that situation.
This is very much a bug in nvidia- graphics- drivers- 340. One can debate if software-properties should or shouldn't purge (though traditionally, the argument was that silent frontends shouldn't purge because the user hasn't explicitly asked for that), but if a removed package's unpurged conffiles cause the system to misbehave, that package needs to be rethought to avoid that situation.