Basically the same thing just happened to me. The difference was I waited a little longer than normal for an opponent to be found.
Once one connected we played 3 holes as normal. After we finished I noticed I had dropped many more slots than I had expected. Then I saw holes 9-12 had been given bogeys, and 12-15 were scored as I had just played.
My goal every showdown is the qualify for the final round. I know with all of the sandbaggers involved I'll never get close to contending .
I'm also aware this type of thing happens every showdown, but I was careful. I have told myself to quit playing showdowns until this seemingly simple issue can be resolved
I should have listened to myself. Lol.