BIP44 recommends to avoid having more than 20 unused addresses in a row, that's why we stop generating new addresses at this point and give a random unused one.  However, you can hack this around by editing MAX_BIP44_GAP constant at https://github.com/byteball/byteballcore/blob/master/wallet_defined_by_keys.js#L25.

Note that signed message is expected to sign the last BB address that you told the bot.  So if you want to avoid all your Bitcoin addresses being associated with the same BB address, your dialog with the bot should consist of repeated series of these 3 steps:
- BB address
- Bitcoin address
- signed message