Based on the information I see in the link below, I would strongly recommend applying for a transit Visa -
https://www.germany-visa.org/airport-transit-visa/
People with a valid visa to Canada, US and other countries are not required to apply for a visa. However, the visa will be cancelled at the PoE when they arrive in Canada. When they leave back, they won't have a 'Valid visa' for Canada. They wouldn't be being their PR cards for the next 3 months. COPR is not a travel document or a visa, it is a document to prove your status in Canada . Indians, who usually require a transit Visa, do not need to apply for one if any one of the following applies -
So, from the information that I see here, Indians might need a transit Visa if they don't have a valid 'Visa' for Canada or a PR card .
This is only my opinion .