Apologies for the issue you’ve been facing with your Championship, I just had a look and have found the issue, you can fix the problem by following these steps:
Edit the event
Go to the Weather section and click the arrow to expand any configured weather presets
In the “Sessions” input of each weather preset, click the “2nd Race” option in the active column to deselect it
Now you can click the “2nd Race” option in the inactive column to select it again
After doing that you should be able to save the event normally. I’m not sure exactly how the events ended up in this state, but I’ve made a note to investigate this and prevent it from happening in a future update to ACSM.