The current way Skytils grabs HP of withers is by grabbing the HP from the vanilla bossbar Hypixel sends to the client, which is based on a invisible wither that has a static unrelated health value, and from my understanding this bossbar data isn't being sent when your out of radius, which causes the bossbar to go invisible and the HP display to not show at times, it also sometimes doesn't update for a few seconds. This the case even if you enable "Boss Bar Fix" feature, which has the description "Attempts to stop bossbars from disappearing."
There's a possible workaround for this which I tested and it works. The way the workaround works is that we grab the HP value from the actual EntityWither which extends EntityLivingBase and implements IBossDisplayData, which both have the methods getHealth and getMaxHealth that don't depend on the BossBarSetEvent.
Unfortunately this workaround will not work for other bossbars that are not related to HP, for example the crystal hollows event bossbars like 2X Powder, cause there's no actual entity for those it is just the invisible wither, for those the issue needs to be fixed on server side, but I am not sure if that is possible, since they would likely need to spawn another invisible wither for each player and make it follow the players. However, with the upcoming Mod API, maybe a way to talk to the server to grab the current Crystal Hollows (and Dwarven Mines) event might be added for that as well.
The feature itself is also wrongly named "Show Necron's HP", after the F7 revamp it should be named "Show Wither Lord's HP" since it works (except when it disappears) for all the wither lords not just Necron.
Due to the discord message limit I will add my example code to the next message, do note that I copied this from my own workaround and had to change some stuff in notepad and I didn't test compiling it.