AppleWatch 1st Generation: black screen but fully responsive

Help please! Apple watch has black screen

• Background info: 

my AppleWatch has had a black screen  for the past 2 days, the last few days I would often find it off, after charging it and using for a little bit.

• Basic trouble shooting on the AppleWatch:

>I forced restart multiple times. Still black.  

>It has been fully charged

>It has been unlocked using iphone nearby;

>Its software was updated;

>It rang when I played the sound with "find my" app;

It vibrated when iMap is in use on my iPhone;

>Neither voice over or screen curtain were on.

• Further trouble shooting

1. Apple Customer Service run a 1st diagnostic just AFTER I updated the watch to OS 8.8.1 but BEFORE updating my iPhone7 : everything looks fine from diagnostic report, but it shows the watch software needs to be updated.
2. Through my iPhone I check for watch updates, none pending but it says "To update the software on your Apple Watch, you first need to update the software on your iPhone".
3. I update my iPhone7 to iOS15.8 and ask Apple to run another diagnostic. Same result: everything looks fine from diagnostic report, but it shows the watch software needs to be updated 
4. I repeated step n.2
5. Restarted both iwatch (force restart) and iPhone, no new updates available anywhere and I am still stuck with a black screen :(

Any suggestion? TIA all!