When using a Happy Ghast, to transport boats, boats with chests, boats containing villagers or wolves or livestock (suspended via leads) and possibly when flying on a Ghast trying to find treasure following a treasure map…the Ghast may begin to gain height and height control becomes uncontrollable.
When following the treasure map (with the map on the hot bar), disconnecting from the Ghast via shift and reconnecting to the Ghast worked to correct this, but when transporting boats etc. it did not work, resulting in my character having to jump off the Ghast from a height and land in water to not die. Afterwards this time I could not find the Ghast, (other times the Ghast was still around) although I may revisit the site to try and find it later.
This has happened to me 3 times over several weeks of play so far, so can be replicated for bug testing purposes…, last time this happened it was, 2 normal boats, 1 boat with a chest and loot in it, 3 tamed wolves in the boats, all being lead with 3 leads on the boats, then I would mount the Ghast with that lot dangling below…then the Ghast will go forwards W key, but will also start to rise and height controls (pointing down) wont work to go down and neither do the keyboard keys for going down..it just steadily rises.
I was live streaming at the time, so can provide video if necessary.
*Note, happened on todays released version of 1.21.100 and versions before.
Additional note, no I was not able to find the Ghast, when I returned to the area.