Plex on Firestick buffering constantly for seemingly random movies

Plex Players Streaming Devices
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Server Version#: 1.29.1.6276
Player Version#: 9.9.0.35519

My PMS is running on a Synology NAS DS1621xs (DSM7) .

I have 5 Firesticks on the network ranging from very old, to 4K and one of those 4Ks is hard-wired. My internal network is 10Gbps (but hard-wired Firestick is 1Gbps). The wireless network is full-mesh AC2600.

When buffering problems come up, I know the first place people jump to is bandwidth, but bandwidth is not an issue here.

I have several movies that this is happening to, but to keep this easier to follow, I’ll only focus on one: Jack Reacher (MP4 / 720p / AC3 / No Subtitles)

No matter which Firestick I play it on, it will always buffer in nearly the exact same spot. It will eventually catch its breath and start playing again, but after 20-30 seconds it will start buffering all over. Even if I seek to the middle of the move, it’ll play for 20-30 seconds and start buffering.

This movie (and all others) play perfectly fine using Windows, IOS, or Android -but- here is where things start getting strange: If I start the movie at the exact same time on the Firestick and another device (like a tablet), they both play perfectly with no buffering. The only difference is that the Firestick changes from Direct Play to Direct Stream.

I did a packet capture on the network traffic (it’s too big to upload), and noticed clusters of TCP-RST from the Firestick to the NAS. This usually precedes the buffering by a few seconds.

When playing alone on the Firestick, it shows “Direct Play” and has the buffering issue.
Playing Alone

Playing on multiple devices changes the Firestick to “Direct Stream” and both devices play perfectly fine.
Playing Multiple

EDIT
As requested, I have uploaded two sets of server logs. The first one is when playing the movie by itself on the Firestick (which has the buffering issue), and the second is when playing alongside another device (which plays normally on both).
Plex Media Server - Playing on 2 devices.log (431.0 KB)
Plex Media Server - Playing on Firestick only.log (428.5 KB)

Firestick Logs
Plex Log.txt (587.2 KB)

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I would suggest the logs from the server would be perhaps more useful here, since playing back the title on multiple devices at once is having an impact on how the content is being served.

Btw, the wired connection on the Fire Stick isn’t 1 Gbps. It’s 480 Mbps max. The official adapter is only a 10/100M Ethernet, and maybe you found a third-party adapter that works with it, but the connection would be limited by the bandwidth of the USB bus of the Fire Stick (which is only USB 2, iirc).

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Btw, the wired connection on the Fire Stick isn’t 1 Gbps. It’s 480 Mbps max. The official adapter is only a 10/100M Ethernet

You are correct, the Firestick is only 100Mbps. I mis-stated that in my post but when I looked on the switch, it was only linked at 100/Full

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