Hi!
Thank you for the details, it helps us a lot.
It is definitely a script doing that, as the placeholder value is completely unset after the page load is completed, I can see that in the console. I have downloaded the rendered front-end and tried to look for code traces where the placeholder might be altered, but I could not find any.
I suspect it might not be only a placeholder related thing, but I am not able to tell, as there are no traces I could follow up on.
I suggest asking the theme author, I am positive he will know for sure, if it is a script built by them. The plugin does not have any active scripts that unsets the placeholder, so it must be something else – but unfortunately I could not find anything.