It is a concern that they also hold. They are working on some sort of fix [0] (I warn you their website is annoying). One quote from their website:

"The LEGO Group aims to send zero waste to landfill by 2025, and in 2018 recycled 93% of all waste from our operations. Additionally, 100% of all plastic waste produced during the moulding of LEGO® bricks was recycled. This includes reusing some of the plastic resin in our own processes, as well as sending some of the waste to suppliers to be recycled and turned into other plastic products."

This is just an overview of their plans but I also remember seeing an interview with their CEO talking about how many different types of materials are trying to address the waste.

[0] https://www.lego.com/en-us/aboutus/responsibility/sustainabi...