In Arabic, if the subject of the verb is irregular muscular plural جمع تكسير مذكر (or in general any irregular plural no matter feminine or masculine it is) then the verb can be in feminine form or in masculine form, there is no disregard to the subject if it's masculine, both are acceptable, and in fact both are use in Quran:
(There are many similar situations in Quran)
However, of course Allah does choose one option for a reason, this is an example of the eloquence of Quran.
There are two opinions about this:
One, which is the most correct and the majority of Arabic language scholars agrees to, is that it depends on what we mean by the subject (which is irregular plural) itself:
- If we mean the individual singulars, then the verb follows the gender of the singular form of the subject. I.e, if we mean that the verb applies to each single one of that plural individually, then we deal with the plural as we deal with it's singular noun, if the singular noun is feminine, the verb becomes feminine, if the singular noun is masculine, then the verb becomes masculine. For example, They Ayah كُذِّبَ رسل means that each single rasoul رسول was disbelieved كُذِّبَ on his own.
- If we mean the whole group of singulars, then the verb is feminized, because the irregular plural جمع التكسير is fake feminine and is dealt with as the real feminine (it's because we refer to Jama'ah جماعة which is feminine), so the verb is feminized. I.e, if we mean that the verb apply to the whole group altogether at one time, then the verb is feminized. And this is the situation in كُذِّبت رسل, it means that some people disbelieve a group of prophets (not one) altogether. In fact this may contain consolation from Allah to Muhammad (pbuh) that don't be sad, if they disbelieve you and you are one, then there are groups of prophets that were disbelieved (I read this meaning in 2 or 3 tafseer books).
The other opinion, which Dr.Fadel Samurrai mentions, but few agree to, is that the masculine form of verb means that the object, which is plural, is only a small group, while the feminine form of verb means that the object is a large group, and also the feminine form of verb may mean stressing the verb, i.e, that the verb is too much applied (in our example, it means that the prophets were greatly disbelieved). However, this opinion is criticized because this rule is broken by some ayas in Quran.
Finally, a context-specific opinion I would like to mention is based on the aya preceding the aya we were talking about:
قَدْ نَعْلَمُ إِنَّهُ لَيَحْزُنُكَ الَّذِي يَقُولُونَ ۖ فَإِنَّهُمْ
لَا يُكَذِّبُونَكَ وَلَٰكِنَّ الظَّالِمِينَ بِآيَاتِ اللَّهِ
يَجْحَدُونَ
Sahih International
We know that you, [O Muhammad], are saddened by what they say. And
indeed, they do not call you untruthful, but it is the verses of Allah
that the wrongdoers reject.
[Al-An'am:33]
This ayah tells that disbelievers don't mistrust Muhammad (pbuh), but rather mistrust the ayat of Allah; the word aya آية is feminine, and the word آيات is regular feminine plural جمع مؤنث سالم which is always dealt like feminine as well.
The opinion says that كذبت رسل in the next aya bears the same meaning, it means that what were disbelieved are not the prophets themselves, but the ayat Allah sent to them, so it becomes "كذبت آيات رسل", and since آيات is always dealt like feminine, then the verb is feminized as well.
Source: I can't mention one source, this is a conclusion of research and many discussions. A link to one discussion is this.
