Slideshow description is opposite of my experience

[Jay Cee]

The description tip under slideshow:
When this option is enabled, the background will dynamically change on new tab pages that have already been opened. If the option is disabled, the background on previously opened new tab pages will remain unchanged, even if the current background has been updated.

I am having the opposite experience. With slideshow unchecked:
I have a new tab 1 with background 'A'
Open another new tab, 2, with background 'B'. But now tab 1 also shows background 'B'.
Everytime I open another new tab, all black tabs show the same background as the most recent tab.

If I enable slideshow it doesnt do that, but technically its hard to tell because the background is changing on its own anyways.

[Browservery Support]

With slideshow on, when "Change background every" time is elapsed, the background will change with "Background change effect" even if you remain on the opened new tab.

With slideshow off, it'll remain unchanged even if "Change background every" time is elapsed.

These rules apply if you opened just one new tab. If you keep opening new tabs, they'll all have most recently changed background. The "Instant" value in "Change background every" means 1 ms, so every time you open a new tab background is going to change and update the background in every new tab. Try increasing  "Change background every"  value to something higher and you'll see the difference.