The third person plural of the passive voice in the present stem has a peculiarity that I noticed a couple of weeks ago (far later than I should have, I might add) and have been curious about ever since: namely, the fact that the vowel of the penultimate syllable in a form such as spondentur (and, in general, all athematic third person plural passive forms in the present stem, including subjunctive and imperfect forms) is short rather than long – or at least, that is the case according to the work on Latin grammar used in my classes (Lateinische Grammatik by Rubenbauer & Hofmann, for reference). I tried looking it up in the work I generally like to reference when researching these types of topics – Historische Laut- und Formenlehre der lateinischen Sprache by Meiser – but was not able to find any information on that specific question, which is why I am now at a loss as to where to look next. If those of you more well-versed in the diachronic phonology/morphology of Latin know the answer to this question or are at least able to point me in the right direction, I would really appreciate it.
As it stands now, I only have the hypothesis that it might have been an analogical vowel shortening, given the shortening of spondent as a final syllable ending in a consonant other than s. From what I have read in the Ars grammatica minor by Dōnātus, for example, it seems that Roman grammarians were more inclined to view passive voice endings as explicit derivates of their active counterparts, in contrast to the practice I encountered in my school education of simply treating them as a separate system of inflectional endings; consider this quote from the aforementioned Ars grammatica minor as an illustration:
[Verba āctīva sunt,] quae in ō dēsinunt et acceptā r litterā faciunt ex sē passīva, ut lego legor.
It might therefore not be that much of a reach that spondentur was shortened in analogy to spondent – or at least, that ancient authors decided to classify the vowel as a short one, even if this did not necessarily reflect spoken language. It is interesting in light of this piece of speculation on my part that spondētur retains vowel length, but maybe also not that unexpected when you consider that shortening that vowel would have required an accent shift, whereas the penultimate syllable in the third person plural is long by position either way.
I would really appreciate any input from those of you with more knowledge on the topic – an explicit answer, a place to look for more information, or even some feedback as to whether my hypothesis is plausible would be a big help. In any case, thanks for taking time out of your day for my question, dear reader.