I wonder if adding autocomplete="off" to the input element changes the behaviour? It is in any case required for sanely using datalists. Otherwise the value selection history will be appended to the dropdown list (observed in Chromium).

Also, datalists still need JS in some cases like when a default value is set. I have the workarounds and browser quirk notes here in the (to-be-launched) new LibreOffice website code: https://git.libreoffice.org/infra/libreofficeorg/+/835a5cc59...

Good news is that a Firefox dev plans to look into improving datalist behaviour, so only a single click would be enough to display the list.