There are several spells that require the target to be "humanoid" such as Hold Person, Dominate Person, etc.
However, if the target is Polymorphed after that spell has been successful, then what happens?
Does the change in form to Beast end the spell effects? Does it suppress the spell until the correct form returns? Does it do something else?
There have been conflicting answers about the topic on Twitter. On one hand, you have Mearls saying the spell would stick:
Nope, restriction applies to targeting - sticks after successful cast
On the other, you have Crawford saying it doesn't stick:
In #DnD, the exceptional trumps the general. (No longer being a valid target trumps condition carryover.)
However, as of the 2019 Sage Advice Compendium release, only the rulings in the SAC are "official"; previously, Jeremy Crawford could tweet "official rulings", but now the SAC says Crawford's tweets are simply "often a preview of rulings that will appear" in the SAC.
The SAC does not mention what happens in this situation. In other words, RaW does not specify what happens.
My favorite ruling about this (makes for fun interactions at the table) is that the spell is suppressed while the target is invalid. It comes from Crawford:
There's no rule governing what happens when a valid spell target temporarily becomes an invalid target. A good rule of thumb is that the spell is suppressed while the target is invalid.
It agrees with other answers about Dominate Beast, for example. Another answer has a different opinion, that spell wording has subtle implications, but I'm not sure the wording is different on purpose, seeing as even Crawford doesn't know about it.
Suppressing the spell when the target is invalid also makes fun interactions at the table, and lets your players get creative. If your poor low-HP friendly Bard is being Dominated by some arcane evil Wizard that has acquired some nasty spell-immunity shields, your friendly Wizard can cast Polymorph or True Polymorph to turn your ally into a T-Rex, which breaks the Dominate Person (as the Bard is now a Beast, and not a Humanoid), and also lets your friendly Bard go on some sort of raging bloody rampage.
In the end, it comes down to DM fiat, and there is no RaW answer.
If the condition/effects imposed by a spell is only valid for a given creature type, the spell effect says so. This is often different (and more general) than the targeting requirement.
This has been clarified in a recent Sage Advice Q&A:
Does Charm Person ends if polymorph is cast on the charmed humanoid, changing its type to beast?
Mike Mearls: nope, restriction applies to targeting - sticks after successful cast #wotcstaff
Spell descriptions state the conditions that will end the spell, and changing form to another type is typically not one of the conditions.
Consider the description of the Dominate Person. (PH 235) It states “You attempt to beguile a humanoid” but then the subsequent effects of the spell refer to the target as “the creature.”
Once the target is under the influence of the spell, they are dominated as long a they remain a creature (and no other condition occurs that would explicitly end the spell).
Contrast this with the effect of the spell Dominate Beast — its spell effect refers to the target as “beast” throughout: not just “You attempt to beguile a beast” but then continuing “while the beast is charmed.” In this case, the spell effect designates the charmed condition, not just the target, relies upon the creature type being a beast.
Charming spells in the PH that target beasts (e.g., Animal Friendship, Dominate Beast) consistently refer to the target as “the beast” in the spell effect, while spells that target humanoids (e.g., Charm Person, Dominate Person) refer to the target as “humanoid” only during the targeting, and then refer to the target as “the creature” and/or “that target.”
There is no contradiction between Mearls’ tweet that Charm Person would persist, and Crawford’s tweet the Dominate Beast would not. Dominate Beast works “while the beast is charmed” (PH 235) and Charm Person affects “the charmed creature.” (PH 221)
For any specific spell, simply check whether the wording of the spell effect changes from the specific creature type to “creature” when it transitions from discussing targeting to delineating the effects and condition imposed, to see if the condition can be dodged by changing your creature type.
