Sounds like a race condition – two or more actions executing at the same time. Can easily happen when using the Schedule an API workflow on a list action. In your case, two actions pull the same completed value, and both increment to the same value.
If making the interval larger on the list processing doesn’t help, maybe you can peel off the action that increments the log into it’s own api, and space them out at random intervals so as to reduce the possibility of them executing at the same time?
Here’s a post on what that might look like: Missing things omitted from list race condition
I think there are also some plugins that can generate the random number instead of using the database.