Breaks when saving any (or zero) changes in theme settings

On a fresh install of Windows Bitnami Drupal 8.3.x, when applying any changes, or even no changes, in the the theme settings, Magazine Lite stops picking up local changes from color-gray.css, and reverts to bootstrap.css defaults, e.g. blue links in titles of teasers.

Here are the steps to recreate the issue:

1. Install Bitnami 8.3.7 to get a fresh install of Drupal 8.3.7.
2. Install Magazine Lite (ML) through the Drupal them installer web interface from the following link: https://ftp.drupal.org/files/projects/magazine_lite-8.x-1.x-dev.tar.gz
3. Set ML as default theme. At this point, the theme is working as expected - local css color-gray.css modifies bootstrap.css and I see dark gray links on my site. Inspecting a link in Chrome confirms this: I can see that the links are colored gray by color-gray.css, which is over-riding the blue link color from bootstrap.css.
4. Go to the ML theme settings and without making any changes, just click "Save Configuration".
5. Clear all caches, then reload site.
6. At this point the theme is partially broken - it is no longer picking up local changes from color-gray.css so what I see is the plain boostrap.css theming, with blue links. Inspecting a link in Chrome shows that it is now derived straight from bootstrap.css.

At this point, the only thing I've found that fixes the issue is uninstalling the theme through the UI and re-installing it. You do not need to delete and re-download the theme files, just uninstall and reinstall from the web UI. At no point in the whole process does color-gray.css get deleted or modified in the file system.

I'm new to Drupal so I'm not sure how to debug this, but here are some things I've tried:

• I checked the Drupal error log; I found no error. I thought maybe I'd see something about not being able to find color-gray.css.
• I checked the theme folder/file tree for modifications during this process; I don't see anything being changed. I also searched in the entire Drupal install folder for references to color-gray.css to see if a reference to it in another file was being deleted. I didn't find anything useful through this.
• Through phpMyAdmin, I searched the database for reference to color-gray.css; I didn't find any.
• I copied the Bootstrap library files to the ML theme folder, and switched to the local Bootstrap library in the theme settings; I didn't find any change in the behavior I described.
• I followed the instructions given on the Github page of the theme to exactly recreate the theme demo site with no additional modules or content added by me; the behavior didn't change.
• I have not tried this in another environment outside of Bitnami on Windows, so it's possible it's just a Bitnami bug.
• Also tried it under Bitnami 8.3.5, same behaviour.