According to the original (KB500) manual, this is a sticky-key feature of the keyboard itself, rather than the OS.
Sticky keys feature for Ctrl, Alt, and Shift (Progrm + modifier + F6)
A “sticky” modifier key may be useful because it doesn’t need to be held down when
performing an awkward combination of keys. Once tapped, a sticky key is active until a nonmodifier key is pressed and released. In fact two sticky modifiers keys can be tapped in
sequence before pressing the key to be modified.
In general, Kinesis recommends this feature be set in your operating system. With the Kinesis
on-board sticky keys feature, if you prefer to occasionally hold down the "sticky modifier,"
only the first keystroke that follows is modified, whereas operating system versions of "sticky
keys" will continue to modify until the sticky modifier is released.
To turn on or off the “on-board programmability” version of sticky keys, press and hold the
Progrm key, add the modifier key to be made sticky, and finally tap F6. To make a modifier
“non-sticky,” repeat this process or perform a total memory reset.
Pressing Left-Shift+Progrm+F6 in this case would specifically solve (but also create!) this issue.