Arelith

On a number of different characters I've tested, equipping 1 katana with nothing in off-hand and having -twohand mode active does not result in a damage bonus at all. Turning it off and on again doesn't work. Equipping different katanas doesn't change it.

I'm unsure if other weapons have the same issue.

I've noticed this as well - with almost all -twohand weapons.

I think something is broken.

I am going to test with divine might as well.

Is this on PGCC or Live Server?
Can you also test Bastard Sword compared to other -twohand weapons such as spears or katanas on both servers and tell me the results?

This issue still persists, and it seems to specifically be the damage bonus not being removed when -twohand is turned off through toggling it with the command.
If one turns twohand off by toggling it with the command and then equips an offhand weapon or shield, the damage bonus is NOT removed.
If one equips an offhand weapon or shield while it is on, the damage bonus IS removed.

With a weapon that cannot be -twohanded on a char with 18 str I have:
1dx+4
+Wepstats
+Wepstats

With a weapon that can be -twohanded:
1dx+6
+Wepstats
+Wepstats

This is on both PGCC and the live server.
The damage was tested with a katana on PGCC and is confirmed to be applying to damage rolls. (16 str character, 1d10+4 with a katana out, was able to hit for 14 even when -twohand was off. Should be 1d10+3 with 16 str when -twohand is off.)
Character sheet shows the same story for spear, but I didn't test the damage numbers with that one.

Thanks for the report. A fix should go live in the days to come!