The tool will likely have a built in 120 ohm resistor, as its expected that what is plugged in to the OBD plug will,
However without the matching one on the ECU or Gateway, its too high, so the voltage shifts a little, having another 120 ohm resistor hooked across the lines should bring it in to spec.
As for your tool causing issues when reading or writing, That can be a number of things, reading should be safe with most tools, however writing may require having a battery charger hooked on to the car and a healthy float voltage, at-least for a number of European brands,
On a hardware level it appears your tool is broadcasting a nice clean signal, and an OBD socket ideally should be a direct connection to an ECU or Gateway, Its possible someone else has hooked in to it, e.g. a toy car logger that has a termination permernantly fitted, Almost every auto tool i have come across has a built in fixed termination of 120 ohms, so if there is another device hooked in with its own 120 ohms, it can pull things out of spec, equally there are a few tools that don't broadcast the silence command before an operation and a lot of Can connected toy products that don't respond accordingly to it.
In general most ECU's have the termination hard wired, so with the ignition off, if you measure anything less than 120 ohms between the can wires with the tool unplugged you may have trouble.