Connection get reset when connecting to a routed IP

Troubleshooting steps
Proxmox host with pfSense router, also hooked to home router through pfSense OPT1 (192.168.10.100). Allow all traffic between LAN and OPT1 in pfSense.
Create Debian 10 container connected to pfSense LAN (192.168.1.5)
Setup a route on Windows with pfSense as gateway (route -p ADD 192.168.1.0 MASK 255.255.255.0 192.168.10.100)
Connect to the container from commandline (CMD/PS): ssh izaac@192.168.1.5 - Disconnects after a few (5-25) seconds, even if active, ie while editing a file.
Connecting the container directly to the home network (192.168.10.203)
Connect to the container from commandline (CMD/PS): ssh izaac@192.168.10.203 - Not disconnected, even if idle.

Please answer the following

"OpenSSH for Windows" version
SSHD isn't available, but AFAIK Windows 10s SSH client is part of OpenSSH since April 2015.
((Get-Item (Get-Command ssh).Source).VersionInfo.FileVersion) = 7.7.2.1

Server OperatingSystem
Debian 10

Client OperatingSystem
Windows 10 Pro (Build 1903)

What is failing
Disconnected, either by the host or by peer. Seems somewhat random.

Expected output
Stay connected for more than a few seconds

Actual output
Connection reset by 192.168.1.5 port 22 or Connection reset by peer

No source files or tests are identified. Start by reproducing the SSH connection through the pfSense-routed topology and compare it with the direct home-network connection, then inspect the available OpenSSH and network diagnostics. Done means the routed connection remains stable during both idle and active sessions.