I recently came across this issue and ended up adding support for data-turbo-stream="false"
by using the following document wide event listener:
document.addEventListener("turbo:before-fetch-request", (event) => {
if (event.target.dataset["turboStream"] == "false")
event.detail.fetchOptions.headers["Accept"] = "text/html, application/xhtml+xml"
})
You use it like so:
<form ... data-turbo-stream="false">...</form>