Description:
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I try to understand why this was implemented.

When you do 
print mb_strtolower("Groẞ"); it becomes 'groß' 
UPPER CASE ẞ becomes LOWER CASE ß

print mb_strtolower("Groß"); will be 'groß'
LOWER CASE ß unchanged

print mb_strupper("groẞ"); will be 'GROẞ'
UPPER CASE ẞ stays unchanged!

But print mb_strtoupper("groß"); becomes 'GROSS'

Why does it not become UPPER CASE ẞ?

Not all words in german can be changed in that way and should only changed if needed.

UPPER CASE ẞ is offical see https://en.wikipedia.org/wiki/Capital_%E1%BA%9E
 

Test script:
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print mb_strtoupper("groß");

Expected result:
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GROẞ

Actual result:
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GROSS