I don’t know whether this is a bug or not, but when applying a Highlight to a part from a localscript doesn’t work. It gets parented to the part, the adornee gets set to the part, but it doesn’t appear, at all.
Dunno if I’m doing anything wrong.
The code is just (for example)
local x = Instance.new("Part")
x.Parent = workspace
local z = Instance.new("Highlight")
z.Parent = x
z.Adornee = x
z.Enabled = true
But it fails to appear.
I think highlight it self is buggy or unfinished as when I try to add a highlight to a part manually on the server side, it won’t work either.
There is also a post about this
Working fine for me, might be a replication issue, try setting the highlight’s ‘Adornee’ property before its ‘Parent’ property. You also don’t need to explicitly set the highlight’s ‘Enabled’ property to true as that is its default value.
Don’t forget that an experience has a hardcoded limit of 31 highlight instances.
Also an important thing to remember is its not 31 highlight instances, it’s 31 highlight groups. For example, you could have like 100 parts and want to have highlight on all of them, only 31 would render, however, if you grouped those parts and put 1 highlight in the group then they will all get highlighted.
By group I assume you’re referring to a model, a highlight’s adornee can be set to a model, but this still means that you have a limit of 31 highlight instances not to be confused with the number of instances a single highlight instance can highlight.
I tried that before. It didn’t solve the issue.
Having the same issues here, I’ve tried creating a new instance through Localscript, changing Adornee, Transparency, and Disabling/Enabling it. It sometimes works but most of the time it doesn’t.
same, I noticed it only works if the highlight is inside the game already and you use the scripts only to change its parent
As A Dev who uses these, ive noticed them to be very buggy, although iirc this is still a really new item and was only recently beta released. so bugs would be expected
nice necropost 
having excessive highlight usage can make this a problem, i had this before and its important to remember that even if highlights arent enabled they’re still contributing to the 31 highlight limit. it also doesnt matter where its parented to