You can watch this 1 minute video showing the issue
https://youtu.be/gccgX7k_h-o

Steps to reproduce:
1- Use any default CC3 or CC3+ character (I used CC3+ in my example video)
2- Export using MergeUV with Unreal preset, you can select All or By type it doesn't matter, the resolution can be 2K or 4K
3- Import to Unreal engine (I used 4.25) you can use auto setup plugin or manually create the materials both lead to the same issue essentially.

Notes: I've tried many things from exporting a charter with nothing on (no clothes, eyes, teeth etc), tried different skins but unfortunately the same result.
CC3+ character is way worse than CC3, CC3 looks a bit acceptable but the issue is there nonetheless when you get close enough.

Expected result is a correct normal map, although everything points to a bad normal map perhaps it could be something else ?

Final notes: Game engine are all about draw calls, you can get away with many things but the most important thing when it comes to optimization is to reduce the draw calls, and when you have an opportunity to reduce the number of materials on a character that's a huge win in terms of reducing draw calls, this why in my opinion the most important features for CC3 to Unreal is Game base character and MergeUV features, and it's also why I strongly suggest to put your efforts in improving and fixing any issues reading these 2 very important features.

Thanks a lot.