I can not upload a folder with a deep hierarchy

I can not upload a folder with a deep hierarchy.

CentOS 7.3
Nextcloud 12.0.2
nginx 1.13.5
PHP 7.1.9


Firefox 55.0.3

When logging in the browser and dragging and dropping the folder, it stops with the remaining time during the upload being displayed.
No errors etc are displayed in the log.

Specifically, it is as follows


When uploading by drag & drop, it stops when b.txt is copied
Also, if you delete a.txt, the problem does not occur and the upload is completed.

The same event could be confirmed on the demo site below.

Problems do not occur with copy other than browser such as client application.

Is there no way to deal with it?

Please also create an issue here: github.com/nextcloud/server/issues