"Little Bar" would suggest it's RM2k3 (Good old ATB/Time Bar). My situation I outlined above would also set you back to the overworld screen by default. As an example:

(Assuming you only start with Lenny)

<> Change Party Members: Lenny Remove
<> Enemy Encounter (whatever)
<> Change Party Members: Lenny Add

That will flash the battle and exit pretty much instantly to the world map. Anyway, post a screenshot of the code where you're taking out the putting in the second character/taking out the first, hopefully in that order.

Can you do a test battle with Lenny just in the party? It might be something wrong with the character, if not (remake that character for testing). Also, make absolutely sure you're actually ADDING the Lenny character at some point, either before/during the battle. (Kinda like making sure the printer is plugged in before calling tech support to drive over and fix it. Still bitter about that one)

If you still can't figure it out you can zip and upload the project folder somewhere and post the link here, and I'll repair it myself if possible. If you don't have anywhere to upload it, PM me and I'll send you my email address and you can send it as an attachment.