Recent performance issues on selected platforms

Server Version#: 1.43.5.11079
Player Version#: latest
I have a set of Plex libraries hosted on a Synology NAS, and several family members are able to access these remotely. At various times they use phones, phones casting to TVs, or plex player apps on their TVs.
Since the recent updates to the player app one family member has reported a whole series of issues with playback; even if they set the resolution to 480p, streaming will stutter, halt, pick up at a point different to where they left off, etc. I suggested trying to downloading a video file to their PC and playing it locally, but the download was subject to the same issues (reportedly it took nearly an hour to download a 720p single episode of a TV series). Other family members are not reporting the same problems. I can watch the server dashboard while they are watching and observe one family member playing 1080p without any pause, while the other one is watching at 480p and regularly stalling and restarting.

Obviously my first question for the unfortunate child is whether there have been any changes to their internet setup and what performance is like on other streaming services. They can stream Netflix and youtube at 4k without any problems at all. So it’s not the network. The degradation has only come in since the player update; before that they were able to watch at 1080p without any problem at all.

I’ve checked the dashboard to note the player/platforms used.
Happy child: HiSense TV/vidaa, Galaxy Tab S7/Android, Samsung TV/Tizen
Unhappy child: Windows PC/windows, Google TV/Android

What can I look at for clues to explain this unacceptable performance? Given that the onset of this problem coincided with the last player update, is there likely to be a fix any time soon?

I have a lifetime Plex Pass so I would be reluctant to move to a different platform, but this is really not acceptable.

Watch one of their sessions again in the dashboard and pay attention to the connection, local, remote, indirect.