Why is Maxim Plokhoi in solitary confinement in a Ukrainian prison?

Ukraine aspires to join the EU.

Solitary confinement is not banned in the EU, but strongly discouraged:

"The EU, through its institutions and human rights frameworks, strongly considers solitary confinement -- especially prolonged or indefinite use -- as inhumane and a form of cruel, inhuman, or degrading treatment."

Ukrainian President Volodomir Zelensky recommended watching the film Earthlings.