These two requests do not return the same results:
https://meta.wikimedia.org/w/api.php?action=query&list=globalallusers&agufrom=d
and
https://meta.wikimedia.org/w/api.php?action=query&list=globalallusers&agufrom=D
The former is incorrect since usernames that begin with a lowercase letter are bugs. To resolve this, the API should internally uppercase the first letter (by using User::getCanonicalName()).
The only alternative is to have every client uppercase the first letter before sending over the request.
This should also be applied to any other endpoint where you can specify the username.
Workaround
T180084: Interaction Timeline V1: The first character of usernames should not be case sensitive