Xiaomi
Send a Group Message (SMS/MMS) auf Xiaomi
Sending one message to several people only stays inside a single thread if the group is created directly in the Messages app. Picking multiple names from the Contacts app during a normal chat usually sends separate individual texts instead.
MIUI does not let you choose SMS or MMS manually. The system checks the content: plain text stays an SMS, while photos, videos or long text blocks switch the conversation to MMS automatically. This is why a group chat can change delivery type in the middle of a conversation without any warning.
To build a real group thread, open Messages ➔ Start chat ➔ Create group. Select the contacts or recipients you want to include, then tap Next. Add a group name and tap Next again, type your message and tap Send.
Group SMS/MMS also depends on your carrier plan supporting it. If replies keep arriving as separate one-on-one texts instead of staying in the shared thread, check whether the group was actually created through this path rather than pulled in from a call log or a contact card.