Inconsistent thumbnails with symlink to network location

I created a symlink to my network share(Synology NAS SMB share). That folder mostly contains video and image files. I used mklink /d link_name \\server\folder_name to create one.

However, when I open folder, thumbnails were invisible; only filetype icons. Now I only see filetype icons.

Again, when I searched inside network folder, thumbnails were correctly displaied in search result window(but not outside search result window).

Are there some settings I missed, or is this the limitation of symlink? It seems this problem only occures inside symlinked network folder.

Things I did:

What do you see in File Explorer?

If Preferences / Folders / Folder Display / Show generic icons for... is turned on, what is it set to (in the drop-down next to the checkbox)?

Is the NAS acting like Cloud Storage or just a normal network share?

Does the problem only happen when you access the folder through a symlink?

Is the symlink on the same NAS drive it's pointing to, or on a local drive?

Preferences / Folders / Folder Display / Show generic icons for... is turned off.

This problem is happen when I access through symlink. When I directly access to network share, thumbnails are properly displayed.

My symlink direction is Local drive -> Network share.

When I access through Windows Explorer, same problem happens. (On first time everything is normal. After that, every thumbnail is not displaying.)

After some testing, I found this; When I access through library(ex.library -> video), everything seems normal. If I access through symlink(ex.D:\video), problem occures. This happens only on DOpus. On Windows Explorer, no luck.

EDIT: It seems it's Windows side problem(Windows forum link, and there's more.). Still, it's wierd that DOpus 'fix' some part of problem... (Accessing through library actually generates thumbnails normally.)

Update: Installing LAVFilter solved video thumbnail issue. (Only on DOpus, not Windows Explorer.)

I'm using SMPlayer, so I didn't install any video codec separately.