I have the same issue. Google Voice to text will spell my son's name with a lower case first letter. If I long press the spelling in the Samsung keyboard, it offers me the option to delete it. After I do that, it acts *exactly* the same way, but if I long press it again then there is no option to delete it.
I tried to create a shortcut to change the lower case spelling to the proper capitalization, but the shortcut logic doesn't accept that as the both of them are spelled the same. Occasionally Google Voice will spell his name completely differently, but even a shortcut for that does not fix it.
I'm guessing that the way text is processed coming from google voice does not trigger the shortcuts the same way that typing would. It's rather frustrating. There does not seem to be a way to update the Google Voice dictionary or personalize it.