Here is a PNG that I converted from .ico format using macOS Preview.app. It kind of flashes when viewed in Safari or Chrome on my Mac. It is not supposed to be animated.
I filed this as a bug (rdar://24844825, mirrored on OpenRadar here) back in February 2016.
It is still happening, and in fact now it happens in Chrome as well.
What is causing this png to be displayed as if it were animated? That "shouldn’t" be possible with PNG!
That's true for a PNG, but not an APNG, which is what you have here.
An Animated PNG is an extension of the PNG format which allows for animation similar to an animated GIF.
Apple adopted the format in 2016 for their animated stickers, which may explain why Preview has the ability to read and write them. It seems it took the different resolutions stored in the ICO and saved them as multiple frames of an APNG.
You can remove the frame you don't want by opening the PNG in Preview, selecting the frame you don't want, and pressing the delete key.