Thanks for your reply.

Unfortunately, nothing helped.

I tried to do this:

$file = new \ElggFile();
$file->owner_guid = $user->guid;
$filename = elgg_strtolower('file/' . time() . 'file.jpg');
$file->setFilename($filename);
$file->filestore_prefix = 'file';

$file->open('write');
$file->write($content);
$file->close();

There was a typo in the previous example not $user->owner_guid but $user->guid.

But that didn't help either.

I'm using Elgg 5.1.9

As Jerome said, setting

$file->owner_guid = $user->guid;

is the reason.

Have you tried NOT setting owner_guid at all?

It also seems you're not setting container_guid in the code of your latest post. I guess you could still set

$file->container_guid = $user->guid;

to preserve the owner information after omitting to set owner_guid.

Thanks, iionly.

I tried doing this with different variations of the code.
For example, this one:

$file = new \ElggFile();
$filename = time() . 'file.txt';
$file->setFilename($filename);
$file->filestore_prefix = 'file';

$file->open('write');
$file->write('Test');
$file->close();

But the file is still saved in the logged in user's folder, but not under its own GUID