Alt_L stopped working as a 'Left Thumb Shift Key' after upgrade to 14.04

[Yuri I. Kornilov]

I prefer using Thumb Shift typing method for Japanese input.
On keyboards with US layout I use Space key as 'Right Thumb Shift Key', and left Alt key (Alt_L) as 'Left Thumb Shift Key' (well, using Alt key as a Thumb Shift is somewhat tricky - you need to press it not simultaneously with a letter key, but in a fraction of a second later then a letter key to avoid acting Alt as a modifier key, but there is no better alternative on US keyboards).
After upgrade to Ubuntu 14.04 Alt_L stopped working as 'Left Thumb Shift Key' - trying to press it simply clears my previous input.
Control_L works as 'Left Thumb Shift Key', but this variant is unusable, because Control keys are too far from the center of the keyboard to be pressed by thumbs.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ibus-anthy 1.5.4-2
ProcVersionSignature: Ubuntu 3.13.0-24.47-generic 3.13.9
Uname: Linux 3.13.0-24-generic i686
ApportVersion: 2.14.1-0ubuntu3.1
Architecture: i386
CurrentDesktop: Unity
Date: Fri May 16 18:44:33 2014
InstallationDate: Installed on 2011-07-19 (1031 days ago)
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
SourcePackage: ibus-anthy
UpgradeStatus: Upgraded to trusty on 2014-05-16 (0 days ago)