By default, macOS'es Quick Look functionality caches previews of all files, even those found on file shares or encrypted file systems. This may not be what you want. Fortunately, you can disable Quick Look caching.

Open Terminal and type in these commands:

qlmanage -r disablecache
qlmanage -r cache
rm -rf $TMPDIR/../C/com.apple.QuickLook.thumbnailcache