BIP 32 child key derivation "stops working" after a certain index (i)
Mycelium HD is BIP 44 compliant. As specified by BIP 44, the address gap limit is 20. This means that after 20 indexes of unused addresses, the wallet expects that no more addresses are used beyond that point. Therefore if you send funds to an address with a gap of more than 20 indexes between the last used address, the wallet software will not check if that address has received any funds.