Some ice doesn't melt ever, though usually these are fairly small unobtrusive bits of ice in the corners of caves or tunnels.

If you find an unmelted piece of ice with a melting requirement that you've already gone past then chances are pretty good that bit of ice is either simply the remains of a larger formation that long since melted or it's just a decorative chunk of ice that is just sitting there as a map feature.

In rare cases the devs have previously used ice to block newly added map locations, e.g. The Toxic Cave area on Planet Prime, in that instance prior to the Toxic Cave area being officially added the ice blocking access had a melting requirement that was achievable in the late game, but it would not actually melt. Once the devs were happy with the new area though they 'fixed' the block of ice so it would actually melt as expected.