Hi Jerry Dammers,
Thanks. Just to clarify, are using the "inline" message response to reply just to the one person in the group?
Or, are you sending a reply to the entire group, and an Android user is replying?
If you are sending the message to the entire group, and when an Android user replies, it is sent as a separate message, not to the group. This is mentioned in Send a group text message on your iPhone, iPad, or iPod touch : "All responses in a group SMS are sent as individual text messages and the recipients can’t see the other responses from the group."
For the "inline" message, are you following these steps?
Reply to a specific message in a conversation
You can respond to a specific message inline in group or individual conversations to improve clarity and help keep the conversation organized.
1. In a conversation, touch and hold a message, then tap Reply .
2. Write your response, then tap .
Tip: You can quickly reply to messages with a Tapback expression (for example, a thumbs-up or a heart). Double-tap the message bubble that you want to respond to, then select a Tapback.
People can respond inline as long as they’re using an iPhone, iPad, iPod touch, or Mac and have iMessageturned on.
Send and receive messages on iPhone
That being said, if the iPhone appears to become unresponsive when attempting to go to a different message thread, or you're unable to navigate any time that you try to complete this task, we'd suggest reaching out to Apple to take a deeper look using: Official Apple Support
Take care!