The Change Timestamp column keeps auto computing even when the column to watch for changes is null

[lala_land]

In my app, I have a “Check In Status” and “Check Out Status” columns. The “Time In” and “Time Out” columns will watch for the “Check In Status” and “Check Out Status” respectively to change before it capture the timestamps.
However, when I use the app to check in only, the “Time Out” column in my google spreadsheet also appear the timestamp.

How can I actually prevent this from happening?

Thank you.