Reproduction steps:
1. Open the attached project "StreamingAssetsBug"
2. Build the project for Android
3. Change the extension of the created file from .apk to .zip
4. Unzip and open the unzipped folder
5. Open the "assets" folder
6. Make sure you are able to see hidden files

Expected result: Both hidden and not hidden files are in the assets folder
Actual result: Only not hidden files are in the assets folder

Reproducible with: 2019.4.33f1, 2020.3.24f1, 2021.1.28f1, 2021.2.5f1, 2022.1.0a16

Workaround:
To fix this issue, you can customize your gradle build.
( Please refer to "Customizing your Gradle build" section in this link https://docs.unity3d.com/2021.1/Documentation/Manual/android-gradle-overview.html)

Steps:
1. Go to "Publishing Settings" section of "Player Settings" (menu: File> Build Settings > Player Settings)

2. Check "Custom Main Gradle Template" and check "Custom Launcher Gradle Template." (The picture in the link is outdated.)

3. Now you can see the path of your gradle build files like below.
"Assets/Plugins/Android/mainTemplate.gradle"
"Assets/Plugins/Android/launcherTemplate.gradle"

4. Find "mainTemplate.gradle" in the path and open the file.

5. Change "aaptOptions.ignoreAssetsPattern" like below. (Find ":.*" in the middle of the pattern list and remove it)
aaptOptions {
....
ignoreAssetsPattern = "!.svn:!.git:!.ds_store:!*.scc:!CVS:!thumbs.db:!picasa.ini:!*~"
}

6. Find "launcherTemplate.gradle" in the path and open the file and change aaptOptions.ignoreAssetsPattern like in the step 5.

Recap: Change aaptOptions.ignoreAssetsPattern in the custimized gradle files
from "!.svn:!.git:!.ds_store:!*.scc:.*:!CVS:!thumbs.db:!picasa.ini:!*~"
to "!.svn:!.git:!.ds_store:!*.scc:!CVS:!thumbs.db:!picasa.ini:!*~"

One more thing, to understand patterns syntax, you can look at the bottom of this page(https://stackoverflow.com/questions/28934333/how-can-i-use-android-aaptoptions)