Having used all of the Image Handler Versions over the years.
I have noticed that the IH3 and IH4 admin tools
Clear Image Cache command has never worked consistently.
It deletes a few files, but never deletes them all and the folders as well.

I tend to play with the images sizes and quality a lot which generates lots of cached images that I need to manually go in and delete. It has always just been an annoyance not an essential feature of this mod.

So if you want this feature to work correctly here is the solution.
Edit file YOUR_ADMIN\includes\functions\extra_functions\functions_bmz_io.php
Find line 25
PHP Code:
    while ($file = $dir->read()) { 
Change to:
PHP Code:
    while (false !== ($file = $dir->read())) { 
Save and upload.
The Clear Image Cache command will now work correctly.