In 1.12.2, using a piston to push frosted ice would make it melt instantly under the correct light conditions. Now, pushing frosted ice under the correct light conditions makes it take longer to begin melting, and it appears to stay at its current age for a while before melting. A good resolution would be either for it to turn into water instantly after being pushed like in previous versions, or for it to begin melting as soon as it has been pushed if so intended. (I'm not sure if the previous behavior was WAI, but it was never reported.)

Here's a video to demonstrate the bug: https://gfycat.com/FluffyCapitalAntelopegroundsquirrel
(It may seem like it in the video, but the pushed frosted ice does not require a block update to begin melting and seems to depend solely on random ticks.)

Here’s another video by ilmango demonstrating the bug in 18w08a: https://youtu.be/3W5fh69D8zg?t=4m27s