UPDATE

I was able to resolve my issue. It has nothing to do with the usage of ownPaymentPubKeyHash. As already correctly stated by others in this thread binding with the help of <- ownPaymentPubKeyHash returns the type PaymentPubKeyHash.

The problem originated from unwrapping the datum incorrectly. And this was caused due to allowing two different types of datum at the script address using the pipe operator. Therefore, I needed to unwrap the datum twice.

My solution looks something like this now:

data ParentDatum = One ADatum | Two BDatum

-- Helper function
getParentDatum :: ChainIndexTxOut -> ParentDatum
getLotteryDatum o = case _ciTxOutDatum o of
    Left _ -> traceError "unexpected datum"
    Right (Datum d) -> do
        let parsedDatum = PlutusTx.fromBuiltinData d :: Maybe ParentDatum
        case parsedDatum of
            Just s -> s
            _ -> traceError "Unknown datum type"



-- inside the off-chain endpoint code
-- {...} 
    where
        isSuitable :: PaymentPubKeyHash -> ChainIndexTxOut -> Bool
        isSuitable pkh o = case getParentDatum o of
            One ADatum{..} -> owner == pkh
            Two BDatum{..} -> owner == pkh
            _              -> traceError "Unhandled datum type"

one :: AsContractError e => Params -> Contract w s e ()
one Params{..} = do
    pkh <- ownPaymentPubKeyHash
    let dat = Stake PPDatum { owner = pkh }
        tx  = Constraints.mustPayToTheScript dat $ Ada.lovelaceValueOf amount
    ledgerTx <- submitTxConstraints validatorInstance tx
    void $ awaitTxConfirmed $ getCardanoTxId ledgerTx

one :: AsContractError e => Params -> Contract w s e ()
one Params{..} = do
    pkh <- ownPaymentPubKeyHash
    let dat = Stake PPDatum { owner = pkh }
        tx  = Constraints.mustPayToTheScript dat $ Ada.lovelaceValueOf amount
    ledgerTx <- submitTxConstraints validatorInstance tx
    void $ awaitTxConfirmed $ getCardanoTxId ledgerTx

second :: forall w s e. AsContractError e => Contract w s e ()
second = do 
    pkh   <- ownPaymentPubKeyHash
    utxos <- Map.filter (isSuitable pkh) <$> utxosAt validatorAddress
    ...
    where
        isSuitable :: PaymentPubKeyHash -> ChainIndexTxOut -> Bool
        isSuitable pkh o = case _ciTxOutDatum o of
            Left _          -> False
            Right (Datum e) -> case PlutusTx.fromBuiltinData e of
                Nothing -> False
                Just d  -> owner d == pkh

second :: forall w s e. AsContractError e => Contract w s e ()
second = do 
    pkh   <- ownPaymentPubKeyHash
    utxos <- Map.filter (isSuitable pkh) <$> utxosAt validatorAddress
    ...
    where
        isSuitable :: PaymentPubKeyHash -> ChainIndexTxOut -> Bool
        isSuitable pkh o = case _ciTxOutDatum o of
            Left _          -> False
            Right (Datum e) -> case PlutusTx.fromBuiltinData e of
                Nothing -> False
                Just d  -> owner d == pkh