Just trying to help out and this is a quick and easy way to check and verify good power and grounds under load to the ecm , which it must have. A lot of guys don't have a good dvom .
Beyond that it may just be a nature of the beast with a modern ecm in these older vehicles. They rely on inputs such as battery temperature and you really would need a scan tool to check to make sure everything input wise is correct and also see if there is any codes related to the charging system.
I would contact Hotwire and see if this is common for this setup.