I tried to use 3-state outputs in a project, but the outputs are not driven at all. Setting T to 0 drives the outputs as expected.

I created a minimal example on a basys3 board with two switches to reproduce this issue.

Looking through the files revealed that .IO_LOC_PAIRS is set to "NONE" for the affected OBUFT, in the .eblif file.
Manually setting .IO_LOC_PAIRS on the OBUFT makes it work as expected. I verified that the output is floating with a voltmeter.

I attached a archive with the example, a f4pga flow config, my conda list output, the .eblif files and yosys logfiles of the working and broken state.
example.tar.gz