Electrum only supports one address type per wallet so that's why it's not seeing the legacy address. You will have to convert your master private key to p2pkh format (xprv) and restore your wallet using that to see the legacy address.

Go to view menu > check console and switch to console tab. Click on the yellow warning box to make it go away. Then type in this


You may be prompted for your wallet password. It'll output the master private key in p2pkh format beginning with xprv. Copy that to the clipboard and go to file menu > new/restore, enter a suitable filename and click next, choose "standard wallet" and click next, choose "use a master key" > paste in the xprv you copied before (don't include the single quotes) and proceed with on-screen instructions for the rest.