I don’t have an answer, but I totally feel your pain! This new stuff has got me all kinds of confused! I too wish it was still the old way!
You may have to copy & paste the address in for now. Mine does that one in awhile too , but other times it works just fine.. I am like you I want the original way back*
Try it this way In Yahoo Classic: You can switch back and forth between Classic and the New Yahoo.
1. Click on the Compose tab to open the Compose page.
2. Click on “TO” and your address book will open on top of the page you are in.
3. Check the boxes (TO, Cc or Bcc) next to those you wish to Forward to.
NOTE: To protect your friends security and privacy, enter the names you wish to hide in the Bcc field.
4. Go to the top or bottom of the list and click on “Insert Checked Contacts.” The address book will close and the names you selected will be in the fields you selected. Click Send.