Iphone 11 apps crashing, safari not working, gallery photos gone and retrieves, restarting. Tried restoring countless times still the same issues. Send help.

I brought my iphone 11 to an Apple accredited / authorized service center and they had my phone undergo diagnostic tests but turned out my phone passed these tests so they just returned it to me.

I have restored this phone countless times and the same issues would still occur.

randomly / intermittent:

• phone restarting by itself
• apps crashing, not launching, not working
• gallery files suddenly gone and retrieving by itself
• Pink pixel-like randomly appearing on screen

please do note that after claiming my phone from the service center, I had my i11 synced with my 6s right after. My i6s is working completely fine so I thought it's okay to use i11 with the 6s backup and files.

but unfortunately, the same issues arise. I think it's my i11, the unit itself?? Since the diagnostic tests said my phone is fine when it's definitely not, what do I do now?

if I return this to the service center they told me that they would only restore the phone since diagnostic tests were passed.

please send help :(