This is a bug, but Blizzard will not be making any punishments to any players or anything like that for using this bug. If they were going to, they certainly would have done it already. Double ice lancing in some form has existed since very early Legion (basically since frost started actually being played). Blizzard has never gone out and punished any players for using double lancing, nor have they ever said they would. So while it is considered a bug, you don't have to worry about getting in trouble for using it.
actually its not strictly a bug. A bug is an issue with the code itself. This is a design problem as the code is doing exactly what it was designed to do.
if you think about it from a mechanical design stand point, I doubt they will actually be able to fix this without completely redesigning the interaction...or just add .5s to the duration of Winters Chill to make double IL standard for everyone without gimmicks...
Given the nerf to Thermal Void adding .5s to Winters Chill from the Flurry should be low risk as well as balancing the loss of DPS from the nerf.