Excessive storage usage of NextCloud client

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[x] This is a bug, not a question or an enhancement.
[x] I've searched for similar issues and didn't find a duplicate.
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Steps to reproduce

I am currently facing the issue that NextCloud is 3.6GB in size on my device, though in "On device" there is only 13 MB. Some context

I recently uploaded a lot of files manually and I think this led to a sudden increase in size.
I deleted the files manually that were shown in "On device" (when it was several GB), with little effect on the actual used storage
The android system shows the usage under "User Data", it is therefore not possible to just clean it as cached files from the system settings.

How can we get rid of the files at all? Why are they not counted as "Cache", which would allow their deletion?

UPDATE: the files are still shown in my gallery and apparently stored in /storage/emulated/0/Android/media/com.nextcloud.client/nextcloud/tmp/niels@xyz/upload/location

Expected behaviour

Synced/Cached local files should show up in the "Cache" section in android settings
It should be possible to delete whatever causes this excessive storage usage

Actual behaviour

Somehow "User data" gets clogged up with large amounts of data (after uploads?)

Android version

Device brand and model

Pixel 6 Pro

Stock or custom OS?

Stock

Nextcloud android app version

3.30.0

Nextcloud server version

29.0.7

Using a reverse proxy?

Yes

Android logs

No response

Server error logs

No response

Additional information

Possible duplicate of #12569

nextcloud / android