Normally in Chumash, when a three-letter word with a segol on each of the first two letters appears at the end of a pasuk (or half-pasuk, with an esnachta), the first segol changes to a kamatz. Thus, for example, in Sh'mos chapter 5 a couple of times: "בֶחָרֶב" in verse 3, "שָׁקֶר" in verse 9. Yet "תֶּבֶן" appears unmodified at the end of several p'sukim (7, 10, 12, 13) in that chapter: why is "תֶּבֶן" different? (Ibn Ezra ad loc. asks this and says he doesn't know the answer; I'm hoping someone else does. It is worth noting, though, that he says this is a property of the word, not of its appearance in these p'sukim.)
Historical linguistically, there are many theories as to the processes that led to what we have in front of us, and in particular give rise to the many "exceptions". I am not qualified to compare and contrast, but look, for example, at chapter 6 of Dr Benjamin Suchard's dissertation, where he discusses various possibilities. I describe one such below.
Pausal lengthening is where a short vowel becomes long in pause. For example, pataḥ becomes qamats, or segol becomes tsere.
Philippi's Law is a hypothetical law where Proto-Semitic (PS) i-vowel turns into an a-vowel in closed, stressed syllables.
Dr Blau's hypothesis is that pausal lengthening preceded the vowel shift (or at least that the vowel shift continued much later than was previously thought), and so a word like PS *milk (=king, cf. מלך), would vowel-shift to PS *malk, but no pausal lengthening would subsequently occur. Thus, when a segol vowel eventually found its way in (*malk > *malek > melek), the pausal and the non-pausal forms were the same. The same would be true of a word like תבן, which comes from *tibn (I think, cf. Akkadian tibnu).
On the other hand, a word like כלב is originally *kalb (I think, cf. Akkadian kalbu), so it would have lengthened to *kālb in pausal. Subsequently, with the segol inserted, we see כֶלֶב and כָלֶב.
It's clear from Dr Suchard's dissertation that Dr Blau doesn't have the last word. Dr Suchard gives many more thoughts on Philippi's Law, and that would be a good place to start for more details.
I've interpreted this in a drash sense. It seems from Pharaoh's and the overseers' commands that they couldn't collect twice as much straw one day and then make twice as many bricks the next day, or form an assembly line with one person collecting straw and someone else making bricks (כלו מעשיכם דבר יום ביומו). They had to spend part of each day collecting straw and the second half making bricks, which must have been much less efficient and was an extra form of oppression.
To symbolize that the straw wasn't the end of the story, it doesn't get a kamatz even when it should at the end of a pasuk.
This doesn't fit with the Ibn Ezra and doesn't work out of the box for the two times the word appears in Yeshaya, but it's a start.