MacOS – Finder image icons don’t display at size 16×16 but load fine at 20×20 pixels

Why would Finder image icons (in column view, list view and icon view in a Finder window) not display an image's content, when no custom icon has been applied. The files are image files (PNG, JPG, TIFF, PSD, etc.) and only the smallest size is affected.

In column view at text size 12 only the generic jpg, or png icon shows, but not the image content itself. If I change the text size (in view options) to 13, the icons slightly increase in size and laod and display correctly.

In icon view the same happens with the icons set to 16×16. As soon as I drag the slider to 20×20, or above all icons show correctly.
List view also displays the generic icons, unless the larger icon size is chosen.

I've tried removing the Finder preferences:

~/Library/Preferences/com.apple.finder.plist

I've also tried resetting the icon services cache here:

/Library/Caches/com.apple.iconservices.store

…and removing those found here:

sudo find /private/var/folders/ -name com.apple.iconservices

Permissions (ls -alOe@) on the /private/var/folders/… (found above) also look the same as on clean, bare-bone installations, where the icons are shown correctly, even when only 16×16 pixels.

In column view as example, if left on text size 13 through a reboot, most icons also show after reducing the size of the text to 12, but turning off "Show icon preview" in the view options and on again, leaves the text size 12 icons generic again, while increasing the size to 13 straight away loads the previews.

Why would 20×20 pixel icons load without issues after every restart and 16×16 just sit there? Are they different caches and/or mechanisms?

Running macOS Sierra 10.12.4 on a Retina MacBook Pro11,3 (mid 2014), 2.5GHz, 16 GB RAM, 500.28 GB Apple SSD, Intel Iris Pro 1536 MB VRAM / NVIDIA GeForce GT 750M 2048 MB VRAM. – Resolution: 2880×1800 set to "Default for Display". No external monitor (or other devices for that matter) is attached.

The issue persists over various updates and even running the Installer Assistant (aka Full Installer) over the installation.
It also exhibits the same behaviour in safe mode, but a second (new) user has no such issues. The Desktop (outside a Finder window) also has no issues displaying 16×16 icons.

My main interest here is not so much the troubleshooting (since the installation in question is quite dirty and it will be down to me to find the problem), but to understand the mechanism that causes the difference in behaviour between loading the smallest icon size and all other sizes above.