[…] and I have configured treesitter (I believe!)

Can you share your config?

I don’t think you need 0.10. to highlight heex correctly, but I can’t verify because I am on 0.10 already.

For reference, I am using nvim-treesitter with the following config:

	{
		"nvim-treesitter/nvim-treesitter",
		build = ":TSUpdate",

		config = function()
			local configs = require("nvim-treesitter.configs")

			configs.setup({
				ensure_installed = { "lua", "vim", "vimdoc", "query", "elixir", "heex", "javascript", "html" },
				sync_install = false,
				highlight = { enable = true },
				indent = { enable = true },
			})
		end,
	},

Issuing the command :TSInstallInfo / :TSModuleInfo also should show heex being enabled.