Wiring issue with 3-way switches and FEIT smart dimmer and T1/T2 lines cross-talk
T2 has some 45V AC voltage on it when T1 is connected to live/hot wire.

Possibly a red herring. I had a similar experience when installing a similar 3-way-compatible &quot;smart&quot; switch (but with a timer feature instead of dimmer). When I asked the electrician about it, they said that if there's any current flowing in wires adjacent to the traveler, especially along long distances, smaller voltages can be induced in the traveler itself.
More important is to verify that you're using the correct configuration. With a 3-way switch, the line voltage comes in at one switch, and then the load is connected at the other switch. These smart switches can be installed at either end, but the wiring configuration is different depending on which end.
The diagram you show is correct for when you want to install the smart switch at the load end, and your summary of the operation of this configuration is consistent with that as well. But that doesn't mean that's actually where you installed the switch.
You should verify that what you believe to be the &quot;load&quot; wire is in fact the load wire, and not the line supply wire, and vice a versa. If it's not, i.e. if the wire you've treated as &quot;load&quot; shows 120V between it and neutral with your multimeter, then you've got the wrong diagram for where you are trying to install the switch.
If that's the case, then you can either install the switch at the other end of the circuit using the diagram you have, or find the diagram that corresponds to installing the switch at the line end of the circuit (it's probably a similar diagram, except that on the smart switch end, the &quot;load&quot; and T1 wires will be connected reverse from what you have).
I have installed a number of this sort of switch in my own house, and have found them to be reliable, but one has to be very careful to follow the instructions precisely.
If you're sure you've identified your wires correctly, and you're sure you're making all the connections correctly, then it's actually possible you have a defective switch. But you'll want to double- and triple-check everything just to make sure, before getting the switch replaced.