That is correct behavior.
You are not supposed to be able to enter a name longer than what can be displayed.
(base name is not displayed only there, but also on many other places and those places, mostly tables, expect a certain maximum length)
Before automated name generation was added, I have explicitly warned about this and the modders wanted (and accepted) the feature anyway, even with this side effect.
PS: if you want to change the name anyway, you can edit the save file manually.