[1.1.81] linux Left control key has stopped working

[mward]

The left control key has stopped working on my Linux version of factorio.
Right control and capslock still work (I have capslock set to work as a control key in System Settings).
Left-Control-C does the same as C while Right-Control-C goes into copy mode as it is supposed to.

xmodmap -pm output is:

xmodmap: up to 3 keys per modifier, (keycodes in parentheses):

shift Shift_L (0x32), Shift_R (0x3e)
lock
control Control_L (0x25), Control_L (0x42), Control_R (0x69)
mod1 Alt_L (0x40), Meta_L (0xcd)
mod2 Num_Lock (0x4d)
mod3
mod4 Super_L (0x85), Super_L (0xce), Hyper_L (0xcf)
mod5 ISO_Level3_Shift (0x5c), Mode_switch (0xcb)

In "Controls Settings" if I press eg "Control+C" to set a key it comes up with "Left Control". If I use the right control key it comes up with "Control+C"

The control keys work fine in xterm, xemacs, Firefox etc.

If I reboot into Windows 10 with the Steam version of Factorio, then the key works
(I don't rebind capslock in Windows)