Any pointers please..?

I have a twilio number which is correctly connected to Manychat. I am also using the Textmagic sms platform to send outgoing SMS messages which include a ‘Keyword’ which should trigger a Manychat SMS flow.

The process has worked several times up until today.  When the required Keyword is entered into the reply SMS message, instead of triggering the Manychat SMS flow, another SMS message (not initiated by the flow)  is sent to the sender which reads “Reply STOP to unsubscribe”.

It looks like I have missed something, but Im not sure what.

Advice appreciated - Cheers :-)