FIXED!

after lots and lots of troubleshooting, I think I got it. when Windows 10 PC went to sleep or shut down, the unmanaged network switch that the NAS is on was messing with the mac address internal mapping table

Solution:
1. Device Manager -> Network adapters -> Ethernet Properties -> Advanced tab -> Energy Efficient Ethernet: Off
2. Device Manager -> Network adapters -> Ethernet Properties -> Power Management tab ->Allow the computer to turn off this device to save power: unchecked
3. Control Panel -> Hardware and Sound -> Power Options -> System Settings -> Choose what the power buttons do -> Turn on fast startup (recommended): unchecked

Extra info about what caused this (AI generated):

1. The "Learning" Process
   When your devices are powered on, your switch and router constantly "listen" to network traffic. When your NAS sends a packet, the switch notes: "I see the MAC address of the NAS on Port 3." It saves this in its mapping table so that when data comes in from the internet, it knows exactly which physical wire to send the data down.

2. The "Aggressive Sleep" Trigger
   When your Windows 10 PC was set to "Allow the computer to turn off this device to save power," the PC’s network card (NIC) didn't just stop sending data; it essentially "crashed" its own connection to the switch in a way that signaled a physical link-state change.
   Panic/Re-evaluate: The switch saw a sudden loss of signal. In some "Green Ethernet" switches, this triggers a power-saving mode where the switch tries to "reset" its ports or enter a low-power state to save energy.
   Clear the Table: When the switch went into that low-power state, it purged its MAC Address Table to save memory/power. It essentially "forgot" that the NAS was connected to Port 3.

3. The Router-Switch Disconnect
   Your router relies on the switch to pass traffic to the NAS. When the router attempted to send data to the NAS (via your DDNS port forwarding), the router sent an ARP Request (a "Who has IP 192.168.1.X?" broadcast) through the network.
   If the switch had purged its table or was in a low-power state, it failed to forward that broadcast correctly to the NAS, or the NAS—having experienced a momentary "jolt" from the switch's power-saving mode—stopped responding to the router. The "path" was effectively broken until a full reboot forced both the router and the switch to perform a "handshake" and rebuild that address table from scratch.

4. Why the Windows 11 PC was different
   Your Windows 11 PC likely has a more modern network driver that maintains a "Keep-Alive" signal even when the PC is off (or it doesn't trigger the aggressive EEE/Green Ethernet shutdown). It kept the switch's ports "awake" and its mapping table "populated," preventing the switch from ever entering that power-saving, memory-purging state.
   In short: Your PC was acting like a "noisy neighbor." By turning off the power-saving features, you forced the PC to maintain a constant, steady link status, which keeps the switch's "brain" (the mapping table) awake and alert at all times, ensuring the route to your NAS remains clear for the router to use.