SUMMARY

If KDE Connect is running, the Android system apparently blocks any new TCP or UDP connection but leaves existing ones connected (unsure, it seems some fail while others stay). The problem disappears when KDE connect is stopped. I'm not sure if this is a KDE connect fault or some misbehavior by Android 15, probably a mix of both.

As a result, the device acts as if it was offline whenever the screen is off, which is a major usability issue.

STEPS TO REPRODUCE
1. Have an Android 15 device
2. Start KDE connect
3. Turn off the display and try any online related activity like sending a message to the locked phone

OBSERVED RESULT
No message is received, no matter how long you wait. When you power the display, Android shows an '!' next to WiFi which returns to arrows in a few seconds and networking works again.

EXPECTED RESULT
The network should always work, only limited by Android power saving features.

SOFTWARE/OS VERSIONS
KDE connect: 1.33.3
Android 15 (OneUI 7.0)
(I picked Android 14 in the report as it is the most recent in the menu, but the issue is with Android 15)

ADDITIONAL INFORMATION
For easier reproducibility, you can try iperf3 running as a server on the phone's terminal. Client is a laptop running Debian:

KDE connect disabled, screen off:
```
$ iperf3 -c 192.168.0.10 -u --connect-timeout 3000
Connecting to host 192.168.0.10, port 5201
[  5] local 192.168.0.12 port 33368 connected to 192.168.0.10 port 5201
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.00   sec   129 KBytes  1.05 Mbits/sec  91  
[  5]   1.00-2.00   sec   127 KBytes  1.04 Mbits/sec  91  
[  5]   2.00-3.00   sec   129 KBytes  1.05 Mbits/sec  90  
[  5]   3.00-4.00   sec   129 KBytes  1.05 Mbits/sec  91  
[  5]   4.00-5.00   sec   127 KBytes  1.04 Mbits/sec  90  
[  5]   5.00-6.00   sec   129 KBytes  1.05 Mbits/sec  91  
[  5]   6.00-7.00   sec   127 KBytes  1.04 Mbits/sec  90  
[  5]   7.00-8.00   sec   129 KBytes  1.05 Mbits/sec  91  
[  5]   8.00-9.00   sec   127 KBytes  1.04 Mbits/sec  90  
[  5]   9.00-10.00  sec   129 KBytes  1.05 Mbits/sec  91  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec  1.25 MBytes  1.05 Mbits/sec  0.000 ms  0/906 (0%)  sender
[  5]   0.00-10.03  sec   969 KBytes   791 Kbits/sec  10.283 ms  221/906 (24%)  receiver

iperf Done.
```

KDE connect active, screen is on:
```
$ iperf3 -c 192.168.0.10 -u --connect-timeout 3000
Connecting to host 192.168.0.10, port 5201
[  5] local 192.168.0.12 port 57485 connected to 192.168.0.10 port 5201
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.00   sec   129 KBytes  1.05 Mbits/sec  91  
[  5]   1.00-2.00   sec   129 KBytes  1.05 Mbits/sec  91  
[  5]   2.00-3.00   sec   127 KBytes  1.04 Mbits/sec  90  
[  5]   3.00-4.00   sec   129 KBytes  1.05 Mbits/sec  91  
[  5]   4.00-5.00   sec   127 KBytes  1.04 Mbits/sec  90  
[  5]   5.00-6.00   sec   129 KBytes  1.06 Mbits/sec  91  
[  5]   6.00-7.00   sec   127 KBytes  1.04 Mbits/sec  90  
[  5]   7.00-8.00   sec   129 KBytes  1.05 Mbits/sec  91  
[  5]   8.00-9.00   sec   127 KBytes  1.04 Mbits/sec  90  
[  5]   9.00-10.00  sec   129 KBytes  1.05 Mbits/sec  91  
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec  1.25 MBytes  1.05 Mbits/sec  0.000 ms  0/906 (0%)  sender
[  5]   0.00-10.09  sec  1.23 MBytes  1.02 Mbits/sec  17.091 ms  13/906 (1.4%)  receiver
```

KDE connect active, screen off:
```
$ iperf3 -c 192.168.0.10 -u --connect-timeout 3000
iperf3: error - unable to connect to server - server may have stopped running or use a different port, firewall issue, etc.: Connection timed out
```

It should be noted that a single iperf3 test runs normally if the test started while the screen is on then switched off, but it would get stuck not declaring results until the phone screen is turned on again (probably waiting to establish another connection for signaling end with the server). Also, the device remains connected to WiFi (screen off+KDE connect) and replies to ARP ping normally, but refuses Transport layer connections in both directions.