I think from a certain size on, they have to take them back. I understand smaller stores not wanting/having to bother. But maybe I'm wrong, after all I almost exclusively buy from REWE, so it's never something I encounter personally.

I encounter this mostly when buying craft beer from a specialized store. These import bottles have Pfand too, but you can't return them to any machine. Have to remember which ones go back to the craft beer shop. Which is kinda obvious though, so I'm not complaining.

I think my most recent experience was a LIDL that wouldn't take a beer bottle from REWE.

Same. My local Lidl doesn't sell any Glas bottle at all, so that's why they don't take it.