I have a Win 10 machine that the DTR fails to toggle. It changes from low to high while avrdude is running, but that does not reset the MCU, then when avrdude gives up DTR goes low (ironic). It does not toggle on my Win 10 machine and is doing the same for both the FT231XS and FT232RL devices (I have a few of each). They are still working on Linux (e.g. they toggle and cause a capacitively coupled reset to run the bootloader). I am thinking that avrdude is not spacing the toggles out enough to be visible or that something is up with the Windows 10 FTDI drivers. Perhaps I'm the only one seeing this (and should feel special).
It sounds like it is toggling just fine, but the opposite state compared to normal. I don't know about that, sounds weird. Perhaps an obscure driver setting. Have you rebooted lately?
oops, yep an LED made me say that... the DTR is active low as you point out, so flip what I said around to: "It changes from high to low while avrdude is running..."
That's what you want it to do, go from high to low while avrdude is running. When avrdude is done, it goes back to high.
Short answer I have the logic flipped, but it was working because in the past it toggled.
The longer answer I send the DTR from an FTDI into an RS485 transceiver to drive a differential pair which runs to a remote device. At the remote device, another transceiver turns it back into a single ended signal that does the reset trick. Some time back I had hacked the remote board to put the RS485 connector on the other side and hot glued the mess in place, but now I see the DTR pair got flipped. So when I send a DTR low the remote gets a DTR high. Anyway, I am fairly sure it was working from Windows 7 with that flipped logic, but I may have just used Linux at that time, which clearly toggles.
I'm going to hack the hack to fix the flipped pair and then it should work.
Update: change of plan I put the hacked board in the trash and modified a later version to do my testing, soldering around the hot glue did not go very well. Now the DTR signal logic is right and it is working as I remember. I'm thinking that I never used Windows with the DTR logic flipped. Perhaps Windows 7 was not toggling, I wonder if it is just with Linux that avrdude toggles DTR.