just to help anyone who finds this via search engine later… .
my solution to getting openhab to show the MQTT changes in the sitemap and report successful mqtt changes in frontail was to replace the MQTT v2 binding with V1 and ensure i used captial letters for commands (i.e. on should be ON). i know the v2 binding is supposed to work (openhab ver 2.5.1), but this was the only change (aside from moving everything from paperui to text files) but it now works flawlessly.
there is so much old and outdated info i spent weeks going through, but this was a really useful link (note the v1 version) MQTT Binding (v1.11) Getting Started 101