Recovery phrases use derivation paths to generate wallets. A legacy derivation path, used by the earliest Solana wallets, supports only one public address, which means no additional wallets can be added.
This doesn't affect how the wallet functions. It only limits the number of wallets under that phrase.
Fix: when importing the phrase, click Advanced and select a derivation path whose last digit has no parenthesis — for example, /0 or /X. These support multiple wallets, while /0' supports only one.