I just tried the following I found from a reddit post and so far, I haven’t run into the buffering issue. It is for local streaming so not sure it applies to you though.
" In case anyone else on the internet winds up here, I had to enable transcoding to fix this . The Fix : To do this, go to Settings , go to the Plex App/Web tab, and in the Home Streaming section, disable Use recommended settings . Then just set the max quality you think your setup can handle locally. Diagnosis : My server was on an old gaming rig, sending to various devices having problems with all of them. The CPU was idle, and the network was capping out at 50-80 Mbps, well below a wired Gb connection but still crazy high for video. Then I checked the videos I was trying to directly stream realizing that there was more content than bandwidth. Not sure why the server limited itself to <100 Mbps when sending directly but transcoding down to X Mbps allows the bursting to buffer at up to 1 Gbps."
i managed to locate the older version on my desktop and installed that.
it appears to work perfectly now.
i also managed to find
plexmediaserver_1.22.3.4392-d7c624def_amd64.deb
plexmediaserver_1.24.0.4930-ab6e1a058_amd64.deb
plexmediaserver_1.24.3.5033-757abe6b4_amd64.deb
plexmediaserver_1.25.2.5319-c43dc0277_amd64.deb
plexmediaserver_1.25.5.5492-12f6b8c83_amd64.deb
plexmediaserver_1.25.8.5663-e071c3d62_amd64.deb
i shall roll through them one by one and check each in turn.
if i hadnt found them however how exactly was i supposed to test this when only the latest version is available?
update: repeatedly playing the same episode in the background while i watch some amazon prime the version i have worked up until 1.25.5.5492-12f6b8c83 which seems to have re-introduced the error.
given the erratic nature of the issue, it is possible that it may be in an earlier version, however after running the same episode through plex for each of the previous versions this is the first one to present an error.
Although even though lots of people seem to be having the same issue, there has been very little movement from plex on it recently. I assume because of their new big shiny toy they released.
