Preview generator does not generate any preview after mass data migrated images

It seems there is a "bug" in Preview Generator app which prevents generation of previews after you performed a mass data migration (copy of images to data folder, perform occ files:scan --all).

I want to import very many image files into a new installation of Next Cloud and I would like to have the previews pre-generated. I do not want to upload several gigabytes via pc client or webdav (after that preview generation works, at least I tested webdav upload and previews for the uploaded files were generated). I think this is why there is this mass data migration available.

After mass data migration, the files are in NextCloud, you could access them via webdav or gui - all seems fine. But Preview Generator "does not see" them. It will not even generate previews for that 3 images which comes along default installation.

Below is the procedure I used for reproducing the issue. I used NextCloud as a snap installation.

1. Fresh install of nextcloud, no previews exists so far:

# ls -al /var/snap/nextcloud/common/nextcloud/data/appdata_ocm286yevx6a/preview
total 8
drwxr-xr-x 2 root root 4096 Aug 22 14:23 .
drwxr-xr-x 7 root root 4096 Aug 22 14:24 ..

2. Install Preview Generator app via GUI, no previews exists so far:

# ls -al /var/snap/nextcloud/common/nextcloud/data/appdata_ocm286yevx6a/preview
total 8
drwxr-xr-x 2 root root 4096 Aug 22 14:23 .
drwxr-xr-x 8 root root 4096 Aug 22 14:26 ..

3. nextcloud.occ preview:pre-generate, no previews exists so far:

# ls -al /var/snap/nextcloud/common/nextcloud/data/appdata_ocm286yevx6a/preview
total 8
drwxr-xr-x 2 root root 4096 Aug 22 14:23 .
drwxr-xr-x 8 root root 4096 Aug 22 14:26 ..

4. copy some jpg to data directory and perform files scan so the files are visible in nextcloud (2017-* were copied there, the rest 3 are coming with default installation)

# ls -al /var/snap/nextcloud/common/nextcloud/data/user/files/Photos/
total 28296
drwxr-xr-x 2 root root    4096 Aug 22 14:30 .
drwxr-xr-x 4 root root    4096 Aug 22 14:10 ..
-rw-r----- 1 root root 1576687 Aug 22 14:30 2017-01-01_123325.jpg
-rw-r----- 1 root root 1623375 Aug 22 14:30 2017-01-01_123328.jpg
-rw-r----- 1 root root 3681799 Aug 22 14:30 2017-01-01_123335.jpg
-rw-r----- 1 root root 3218276 Aug 22 14:30 2017-01-01_123725.jpg
-rw-r----- 1 root root 3628021 Aug 22 14:30 2017-01-01_123742.jpg
-rw-r----- 1 root root 3895594 Aug 22 14:30 2017-01-01_123853.jpg
-rw-r----- 1 root root 3029542 Aug 22 14:30 2017-01-01_175627.jpg
-rw-r----- 1 root root 3027073 Aug 22 14:30 2017-01-01_175717.jpg
-rw-r----- 1 root root 2900706 Aug 22 14:30 2017-01-01_175730.jpg
-rw-r--r-- 1 root root  819766 Aug 22 14:10 Coast.jpg
-rw-r--r-- 1 root root  585219 Aug 22 14:10 Hummingbird.jpg
-rw-r--r-- 1 root root  955026 Aug 22 14:10 Nut.jpg

# nextcloud.occ files:scan --all
Starting scan for user 1 out of 1 (user)

+---------+-------+--------------+
| Folders | Files | Elapsed time |
+---------+-------+--------------+
| 5       | 16    | 00:00:00     |
+---------+-------+--------------+

5. now we have 16 files in Next cloud - if I would go to GUI now I would see all the images there, but I will not do it and perform preview generation:

# nextcloud.occ preview:pre-generate
#

6. No previews were generated:

# ls -al /var/snap/nextcloud/common/nextcloud/data/appdata_ocm286yevx6a/preview
total 8
drwxr-xr-x 2 root root 4096 Aug 22 14:23 .
drwxr-xr-x 8 root root 4096 Aug 22 14:26 ..

7. Copied 2 files via webdav and tried initiate preview generation, all previews for that 2 images were generated:

# nextcloud.occ preview:pre-generate
# 

# ls -al /var/snap/nextcloud/common/nextcloud/data/appdata_ocm286yevx6a/preview
total 16
drwxr-xr-x 4 root root 4096 Aug 22 14:36 .
drwxr-xr-x 8 root root 4096 Aug 22 14:26 ..
drwxr-xr-x 2 root root 4096 Aug 22 14:36 124
drwxr-xr-x 2 root root 4096 Aug 22 14:36 125

# find . -type f
./125/128-96.jpg
./125/256-256-crop.jpg
./125/683-512.jpg
./125/32-32-crop.jpg
./125/1024-1024-crop.jpg
./125/2048-1536-max.jpg
./125/1536-1536-crop.jpg
./125/85-64.jpg
./125/43-32.jpg
./125/512-384.jpg
./125/64-48.jpg
./125/1365-1024.jpg
./125/32-24.jpg
./125/64-64-crop.jpg
./125/512-512-crop.jpg
./125/341-256.jpg
./125/1024-768.jpg
./125/256-192.jpg
./125/171-128.jpg
./125/128-128-crop.jpg
./124/128-96.jpg
./124/256-256-crop.jpg
./124/683-512.jpg
./124/32-32-crop.jpg
./124/1024-1024-crop.jpg
./124/2048-1536-max.jpg
./124/1536-1536-crop.jpg
./124/85-64.jpg
./124/43-32.jpg
./124/512-384.jpg
./124/64-48.jpg
./124/1365-1024.jpg
./124/32-24.jpg
./124/64-64-crop.jpg
./124/512-512-crop.jpg
./124/341-256.jpg
./124/1024-768.jpg
./124/256-192.jpg
./124/171-128.jpg
./124/128-128-crop.jpg

Reproduce the issue with the documented snap installation steps, including files:scan --all and preview:pre-generate, and compare the copied files with the two uploaded through WebDAV. Check the preview output under the documented appdata.../preview path; done means pre-generation creates previews for files added by mass migration as it does for WebDAV uploads.