Relates to MC-12699.
The bug
If an animated png file is used as a texture, the game will only render the first frame of the file, and all following frames will be ignored.
How to reproduce
A resource pack is attached that replaces the glass texture with this public domain animated PNG file: https://commons.wikimedia.org/wiki/File:Animated_PNG_example_bouncing_beach_ball.png
Download and apply the attached resource pack
Look at glass
Expected results
The texture would be animated.
Actual results
It is not.
How to fix
Much like MC-12699, this issue most likely arises from a broken or incomplete implementation of the png specification.
Whether these files should be converted over to Minecraft's native animation format is another question entirely.
This is not a bug, animated textures need a corresponding .mcmeta file in order to be animated.
In your case, apart from glass.png, you need glass.png.mcmeta (with correct instructions inside - look for how it's done in fire_layer_0.mcmeta file, or sea_lantern.png.mcmeta, or any other animated block's case).
This issue can be closed.