Some kids really love going to daycare while mom is home on mat leave. They quickly learn that home becomes routine of feeding/diaper changes/naps and all the laundry and all that stuff. Mom is often busy and occupied and baby is the focus. Going to daycare to see their friends and escape 'baby land' can be a welcome relief. It will obviously depend on many factors but I wouldn't jump to the conclusion that the child will be upset that mom is home with baby.

At 20months the child may just be hitting another separation phase. My 2yr old definitely started one a little before 2yrs. I think they suddenly get smarter and realize they are apart from mom and can't handle it. Keep consistent, don't feed into the crying too much and hopefully it passes soon enough.

Good luck!