Messages suddenly fail to send to specific contacts (one-way communication issue)

[PEnzym]

Checklist

• I can reproduce the bug with the latest version given here.
• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the bug report will be dismissed otherwise.
• This issue contains only one bug.
• I have read and understood the contribution guidelines.

Affected app version

1.9.1

Affected Android/Custom ROM version

Android 13

Affected device model

Galaxy S10 Lite

How did you install the app?

F-Droid / IzzyOnDroid

Steps to reproduce the bug

At some point, the app completely loses the ability to send SMS/MMS messages to specific contacts. When attempting to send a message, it fails, but I can still receive incoming messages from those exact contacts without any issues (one-way communication).

Expected behavior

Messages should be sent and delivered successfully to all contacts, as long as the network connection is stable and incoming traffic works.

Actual behavior

1. Open a conversation with a specific contact.
2. Type a message and hit send.
3. The message fails to deliver.
4. Have the same contact send a message to me — their message is received successfully.

Screenshots/Screen recordings

No response

Additional information

No response