Moz rank while quite useful, isn't considering completely reliable. There are domains that have faked their Moz domain and page trust authority.
If you're losing Moz rank, it doesn't necessarily mean that you will also lose your rank in Google search results.
http to https usually requires 301 redirects, and 301 redirects have been known to transfer almost all of the link juice from the previous address. Reputable sources say that 301 redirects pass between 90%-99% of the link juice. The loss of link juice could be causing the loss in Moz rank. But it's recommended that sites upgrade to https. So despite the link juice loss, the https upgrade should boost your rankings. Thus the upgrade should outweigh the link juice loss.