I've converted this outfit from young adult to teen. I put baked image from Blender into multiplier to check how it will be, and it shows like above. Not sure how to fix it. Any advice will be appreciated.

Looks to me like the edges along the UV seams might have gotten merged, did you by any chance remove doubles?
You should be able to see the messed up UVs if you reimport the exported mesh

Looks like you didn't split seams, and the UV vertices are crossing on the map. I love you Smug, but still use the older Blender tools because many of my meshes are too much for me to split manually with the latest tool. On rare occasion, I get a stray vertices on my UV that didn't split, and go back to rip just that one.

Somehow I solved this problem Thanks for giving me advice!

You should be able to mostly automate splitting the edges, you can simply create seams from UV islands, select one edge marked as seam, select similar >> seam, and split the edges(with 'V' I think).
The reason for not just doing it automatically is because I don't want my tool to recalculate vertex IDs behind the user's back

I never had any issue with vertices relocating, only the rare time when they did not split correctly, which shows similar to the issue here. On my end, Meshing tool kit does not seem to fix neck, waist, ankle seams on teen female, so I export as .geom, as your seam fix works perfectly. On those (usually only one) vertices, I rip and move into position on the UV map manually. This happens sooooooooooo rarely, that doing an entire mesh manual split isn't worth the efoort for me, as your tool works perfectly 99.9% of the time. )

Honestly, some of my meshes have so many parts and take up an entire UV map. ))) As I said, I could never do anything without your tool! Not sure the img upload worked?