You hear different things. But some PAs do get approvals, and sometimes at parts in process where it seems ntohing has changed. Definitely chances seem to be higher where they have looked at the app and made at least preliminary assessment that the relationship is genuine (which I think basically is pre-arrival but not only pre-arrival).Is it worth applying even if the PA has no ties to their home country? or will that be a waste of time and effort? -- Assuming PA passed eligibility and got pre-arrival, and the only thing in process atm are background checks.
Then, it sometimes seems that a trv application in process spurs the final decision on the PR app - probably only if just somewhat lucky in time (the bureaucratic bit being - if the same officer has to do a sign-off on the trv, and all the officer needs to do on the PR app is a few steps and it saves them time, then why not). Of course that could all just be random and we don't know.
The one thing we know: 100% of those who don't apply for TRVs don't get them.
[Note, I'm not advocating applying for trvs hundreds of times - but if you can handle the cost and time and want to be together and/or depart where you are, then it's worth a try or possibly three. We do sometimes have posters here complaining they can't do certian things, be together, make concrete plans ... and they have not even applied for a TRV.]