To reproduce:

1. Open Unity and create a New Project
2. Right click on Project view and press Create > Folder
3. Name it "ABC"
4. In folder "ABC" create Script, Shader or something else and name it "Gebäude" - You must write this name with a German keyboard
5. Open Terminal and write - echo <path to file which name is "Gebäude"> | od -cx and press Enter
6. Observe that You get these results:

Assets/Graphics/SceneElements_A1/Building/V2 Gebäude_10.png.meta
0000000 V 2 G e b ä ** u d e _ 1 0 . p
3256 4720 6265 a4c3 6475 5f65 3031 702e
0000020 n g . m e t a \n
676e 6d2e 7465 0a61
0000030

7. Return back to Unity and Duplicate "ABC" folder
8. Open Terminal and write - echo <path to file "Gebäude" which You duplicated> | od -cx and press Enter
9. Observe that You get these results:

Assets/Graphics/SceneElements_A2/Building/V2 Gebäude_10.png
0000000 V 2 G e b a ̈ ** u d e _ 1 0 .
3256 4720 6265 cc61 7588 6564 315f 2e30
0000020 p n g \n
6e70 0a67
0000024

Expected results: The Duplicated and Original file name are the same
Actual results: The Duplicated file name is different - containing another ä encoding

Notes:
- If You take any file, rename the file to Gebäude and inspect it via Terminal You will observe that charset is still correct

Reproduced on Unity 2017.4.0f1, 2017.4.17f1, 2018.1.9f2, 2018.2.19f1, 2018.3.0f1 and 2019.1.0a11