Heat Metal may be cast on "a suit of heavy or medium metal armor".
Plate Armor is a suit of heavy metal armor. It "consists of shaped, interlocking metal plates to cover the entire body. A suit of plate includes gauntlets, heavy leather boots, a visored helmet, and thick layers of padding underneath the armor." We know from their being referenced elsewhere that gauntlets and helmets are objects in their own right.
A druid, on her turn, casts Heat Metal on a Knight's armor. The Knight takes 2d8 fire damage. Regardless of the results of his Constitution Save, he cannot 'drop' his armor on the druid's turn.
On the Knight's turn, he removes his helmet (free object interaction) and takes off his gauntlets (use an object action) and casts them aside. In just 99 more rounds he can finish doffing his armor.
On the Knight's Squire's turn, his Squire, who is in leather armor, picks up the Knight's helmet (free object interaction) and puts it on (use an object).
On the druid's next (subsequent) turn, she can "use a bonus action...to cause this damage again."
If the druid does so, what happens, and why?
She cannot cause damage to anything, because the "suit of armor" as it existed is no longer an "object" (and perhaps it is now three different objects).
She can cause damage to the Knight (through the remaining armor) and the Squire (through the helmet).
She can damage the Knight or the Squire, but not both.
Something else / none of the above.