If you connect the controller, and then it disconnects from the Wii U due to going from the home menu to Breath of the Wild (for example), then the controller needs to power-cycled to re-connect - you can't just press any button, like you would when using this controller with other systems, or when using the Wii U Pro Controller on the Wii U.
Edit 2020-01-27: I can confirm this still happens with Bloopair v0.6.0 and 8BitDo SN30 Pro+ firmware v5.04.
If you connect the controller, and then it disconnects from the Wii U due to going from the home menu to Breath of the Wild (for example), then the controller needs to power-cycled to re-connect - you can't just press any button, like you would when using this controller with other systems, or when using the Wii U Pro Controller on the Wii U.
Edit 2020-01-27: I can confirm this still happens with Bloopair v0.6.0 and 8BitDo SN30 Pro+ firmware v5.04.