SMS from known sender showing as 'unknown sender'

[AC Question]

My wife has Huawei P8 on Giffgaff UK.
She had a problem with the stock messenger where some received SMS weren't being received.
I therefore installed Google Messenger which seemed to fix the problem.
However, since then, occasional SMS are being received, but instead of going into the conversation for that person, they're going into an 'unknown sender' thread - even through the individual messages in that thread do actually display who they came from (there's a variety of senders, all of whose messages do generally do go to the correct thread).
When I check back to the stock Huawei Messenger, the messages that in Google appear in this 'unknown sender' thread don't appear at all there.

Does anyone have a clue what's going on?

 

[Bensonmojo]

I am having basically the same problem with my Huawei GX8 on StraightTalk (At&t). The sender unknown thread would show up in Google Messenger, but after deleting it, now it only shows up in Google Hangouts. Not in stock messaging app or Textra which I downloaded. The texts are from my contacts but don't appear on their thread.

 

[pmich]

Did you ever get this fixed? Now that Google Messenger (android messaging) is using RCS on my carrier, I'd prefer to use that over the stock Huawei messaging app.