SharePoint 2016 Changes Content of HTML Files on Download

I have an environment with a bunch of HTML files that are being inserted into Content Editor Webparts.  The content database was copied and upgraded from 2013 to 2016.  When we download an HTML file from the Site Assets library, SharePoint changes the relative URLs for images INSIDE the HTML file from the Site Assets library to the Pages library.  Date/Time stamp does not change.  It does not create a new document version.  I have replicated this, so I know it is happening.

Steps:

• SharePoint Page sitting in the Pages library has a content editor web part loading an HTML file sitting in the Site Assets library.
• They browse to the Site Assets library and download the HTML file.
• SharePoint changes the link to images INSIDE THIS FILE from 

../SiteAssets/Img/avatar.jpg
to
Img/avatar.jpg

which effectively looks for the folder and file inside the Pages library.

• The downloaded file AND the one in SharePoint are now changed as soon as they click the download button.
• The SP Page loading the HTML file now shows a bunch of missing images because the link changed to a non-existent folder.
• The HTML file does not show a date\time stamp change or create a new Version of the file in the library.
• The content is just changed.

Thanks,

Jeff