Addon cannot access some files through moz-extension://... when installed from the store

EDIT: I found a workaround by placing all files into the root folder instead of the assets subfolder. This is, of course, not desirable, but works for now.

Hi,
I’m new to developing firefox extensions.
My extension has some .png files under an ‘assets’ sub folder.
Those are listed under web_accessible_resources in the manifest.json.
When I load the plugin locally through about:debugging, all icons show up as expected.
When I open the extension installed from the store (same code), it can’t access them with a “File not found” error when trying to open the correctly generated url moz-extension urls from runtime.getURL().
Looking into the .xpi file under my user profile, I can see all folders/files are where they should be. I can access every other file in that xpi through the moz-extension urls, except the ones in the ‘assets’ folder.
All files are listed in the META-INF/cose.manifest and manifest.mf files.

Any hint to what i’m doing wrong or how to further debug this would be much appreciated.

The extension in question is

EDIT: opening moz-extension://[id]/ shows the following list (including the files under assets):
300: jar:file:///C:/Users/jonas/AppData/Roaming/Mozilla/Firefox/Profiles/ps0iv5nq.default/extensions/%7B878c5814-8eb9-4262-9b4b-f46e74dd7cfa%7D.xpi!/
200: filename content-length last-modified file-type
201: 3rdpartylicenses.txt 17343 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: META-INF/ 0 Mon,%2031%20Dec%201979%2023:00:00%20GMT DIRECTORY
201: assets\MadeByTheCommunity_White.png 551733 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: assets\ccugame-16.png 527 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: assets\ccugame-32.png 960 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: assets\ccugame-48.png 1444 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: assets\ccugame.png 2765 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: assets\s42.png 8679 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: assets\sc.png 2269 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: assets\tophat.png 737 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: background.js 2088 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: favicon.ico 2765 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: index.html 6840 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: main.36304f06ff07d39ebdc0.js 670096 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: manifest.json 1131 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: polyfills.bf99d438b005d57b2b31.js 36864 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: runtime.359d5ee4682f20e936e9.js 1485 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE
201: styles.da3d24e2ce5437c1f846.css 71535 Thu,%2029%20Nov%201979%2023:00:00%20GMT FILE