iPhone: sending iMessage that supposedly is not delivered

My niece has an iPhone 8 that is acting. When you send an iMessage to another user, the message is sent and received. However, she does not see a status (that is, delivered or read). Instead, after a few minutes, you receive a message indicating that the message could not be sent / delivered. The original message has a red exclamation point next to it.

As mentioned, the message was nevertheless sent and received. This happens when messages are sent to any iMessage user, regardless of whether they are sent over Wi-Fi or mobile networks. It even happens, after the recipient has responded.

I have rebooted the device, logged out of iCloud and restarted the network configuration. Nothing seems to fix the problem.

Does anyone have an idea of ​​what might be the cause?

Thanks for any advice

PS: She is running iOS 12.3.1 and it was not always like that. It started happening a couple of weeks ago. It happens that approximately 95% of the messages you send. There is no real pattern for the few messages that are sent successfully, often the following message will already throw an error.