Special characters remove characters in front

As you can see in the video below, special characters, for example the German umlauts ä, ü and ö, but also ß, as well as quotation marks, currency symbols and so on remove normal letters that are placed in front of them. 

Logically, the problem does not occur if the special character is at the beginning of the message, because no preceding characters can be deleted.

This makes writing altogether very laborious.

Thanks in advance for resolving the problem.