When calling world#getHighestBlockYAt() in the nether, it will always return the top of the bedrock ceiling. The reason I believe this is a bug as this information isn't necessarily useful, as (afaik) world#getMaxHeight() gives you the same information.

This example takes the value returned by the method and adds one to get the block above it.

The last version in which I tested this was on Bukkit 1.7 (Legacy version of this plugin) where it gives the highest Y coordinate properly. I cannot confirm how long this has been the case because of this, but I'm willing to bet that it's a new bug introduced with Vanilla 1.16.2 as it's the nether update.