UnityIAP iOS; prices shown, but sandbox purchase fails for Testflight build. Works fine for xcode development build

I'm having some trouble with sandbox UnityIAP, only for TestFiight builds. Everything works fine when I build and install to device via xcode.

I've tried using real itunesaccount users, and sandbox users.

Products are received fine, I can even see the product prices in game. Only when I click to purchase, does it fail, with the following logs.

I'm using Unity 5.6.1f1, updated to latest version of UnityIAP

 myapplication[377] <Notice>: UnityIAP:UpdatedTransactions
 itunesstored(iTunesStore)[128] <Notice>: ISDevice: Taking power assertion: com.apple.itunesstored.purchase
 accountsd(AccountsDaemon)[106] <Notice>: "<private> (<private>) received"
 itunesstored(Accounts)[128] <Notice>: "The connection to ACDAccountStore was invalidated."
 itunesstored(BiometricKit)[128] <Notice>: identities: 0x0((null))
 itunesstored(BiometricKit)[128] <Notice>: identities: -> <private>
 symptomsd(SymptomEvaluator)[142] <Notice>: no stop because the ticker was not found
 itunesstored(iTunesStore)[128] <Notice>: ISStoreURLOperation: Started request at [520709912.24] for <private>
 itunesstored(StoreServices)[128] <Notice>: SSVCookieStorage: Feature Enabler removing ITFE cookie
 itunesstored(StoreServices)[128] <Error>: Failed to add Anisette headers to request <NSMutableURLRequest: 0x105d08630> { URL: https://p100-sandbox.itunes.apple.com/WebObjects/    MZFinance.woa/wa/inAppBuy }. error = Error Domain=SSVFairPlayErrorDomain Code=-45061 "(null)"
 itunesstored(BiometricKit)[128] <Notice>: identities: 0x0((null))
 itunesstored(BiometricKit)[128] <Notice>: identities: -> <private>
 itunesstored(iTunesStore)[128] <Notice>: ISStoreURLOperation: Sending headers for <private>:
 <private>
 itunesstored(CFNetwork)[128] <Notice>: TIC Enabling TLS [80:0x101bbbc30]
 itunesstored(CFNetwork)[128] <Notice>: TIC TCP Conn Start [80:0x101bbbc30]
 itunesstored(CFNetwork)[128] <Notice>: Using <private> for setup of Connection 80
 itunesstored(libsystem_network.dylib)[128] <Notice>: network_config_register_boringssl_log_debug_updates Registering for BoringSSL log debug updates
 itunesstored(libsystem_network.dylib)[128] <Error>: network_config_register_boringssl_log_debug_updates Failed to register for BoringSSL log debug updates
 CommCenter(libATCommandStudioDynamic.dylib)[78] <Notice>: QMI: Svc=0x03(NAS) Ind MsgId=0x0051 Bin=['01 1F 00 80 03 05 04 3C 03 51 00 13 00 14 06 00 CB F3 AA FF 06 00 A2 01 00     32 A4 03 00 01 64 00']
 itunesstored(CFNetwork)[128] <Notice>: TIC TLS Event [80:0x101bbbc30]: 1, Pending(0)
 CommCenter(libATCommandStudioDynamic.dylib)[78] <Notice>: QMI: Svc=0xe2(BSP) Req MsgId=0xe002 Bin=[<private>]
 CommCenter(libATCommandStudioDynamic.dylib)[78] <Notice>: QMI: Svc=0xe2(BSP) Resp MsgId=0xe002 Bin=[<private>]
 itunesstored(CFNetwork)[128] <Notice>: TIC TLS Event [80:0x101bbbc30]: 2, Pending(0)
 itunesstored(libboringssl.dylib)[128] <Error>: Function boringssl_context_get_peer_npn_data: line 1214 Peer's advertised NPN data is NULL or empty
 itunesstored(CFNetwork)[128] <Notice>: TIC TLS Event [80:0x101bbbc30]: 20, Pending(0)
 itunesstored(CFNetwork)[128] <Notice>: TIC TCP Conn Connected [80:0x101bbbc30]: Err(16)
 itunesstored(CFNetwork)[128] <Notice>: TIC TCP Conn Event [80:0x101bbbc30]: 1
 itunesstored(CFNetwork)[128] <Notice>: TIC TCP Conn Event [80:0x101bbbc30]: 8
 itunesstored(CFNetwork)[128] <Notice>: TIC TLS Handshake Complete [80:0x101bbbc30]
 itunesstored(CFNetwork)[128] <Notice>: <private> now using Connection 80
 itunesstored(CFNetwork)[128] <Notice>: <private> sent request, body S
 itunesstored(CFNetwork)[128] <Notice>: <private> received response, status 200 content K
 itunesstored(CFNetwork)[128] <Notice>: <private> response ended
 itunesstored(iTunesStore)[128] <Notice>: ISStoreURLOperation: Received 200 response: [0.83s] <private>
 itunesstored(iTunesStore)[128] <Notice>: ISStoreURLOperation: Received headers for <private>:
 <private>
 itunesstored(iTunesStore)[128] <Notice>: ISStoreURLOperation: Finished loading: [520709913.08, 0.84s] <private>
 itunesstored[128] <Notice>: PurchaseProtocolDataProvider: Error handler resolved with type: 0
 itunesstored[128] <Notice>: PurchaseOperation: Purchase failed with code <private>: <private>
 itunesstored[128] <Notice>: PurchaseOperation: storeCorrelationID: <private>
 itunesstored(iTunesStore)[128] <Notice>: ISDevice: Releasing power assertion: com.apple.itunesstored.purchase
 itunesstored[128] <Notice>: [AppReceipt] No receipt data for: <private>
 itunesstored[128] <Notice>: StoreKitClient: Buy failed with error: <private> for payment: <private>
 myapplication[377] <Notice>: UnityIAP:UpdatedTransactions
 myapplication[377] <Notice>: UnityIAP:PurchaseFailed: 0