> not 100%, but close enough to warrant investigation.
In the singular, this is the result of shortening of long vowels in
jussive/preterite forms in what would be CVVC syllables before early
word-final short vowel loss (e.g. -CVVC# -> -CVC#), as reflected in
the Masoretic text, and discussed in Chapter 4 of my dissertation,
GKC p. 131, etc. As for the plural, I have no idea at all, unless
the distinction between imperfect yaquum and jussive yaqum in the
singular was carried over into the plural as yaquumuu vs. yaqumuu
(where shortness of the vowel of the second syllable of jussive
yaqumuu is due to analogy), and similarly with the consecutive
imperfect. But such hypothetical analogous forms as *yaqumuu are not
reflected in the masoretic text.