Thank you for your post.
It sounds like you have a OnObjectExit script trigger, as this would execute before the tab completes, and therefore leaves you at the enterable field again. At the end of your script, include the script step "Halt Script". This will cancel the following tab.
Let me know if you need additional clarification.
You should also be able to fix this by removing the field from the tab order for this layout. And if there is no tab order, commit records will also remove the focus from the drop down list and yet keep the focus from automatically returning to this field and thus deploying the list all over again.
The first suggestion worked. The second from Phil dod not seem to change anything. Either way -thanks!