Just did the update yesterday on my Tab S4. During update I got the scrambled screen. After a reboot it was gone and confirmed update was successful. Rebooted again and just like you after I enter the PIN it starts flickering and there are lines on the screen. Wait a few minutes and hit the power button and it loads as normal. This only happens after entering the PIN and I can recreate this over and over. I did factory reset on the device and just started over and the issue is still there. I think the update screwed the pooch...
This only happens the first time I enter PIN after reboot. Once it kicks over I can lock the screen and reenter PIN with no issues.
The same thing exactly happened with me, I tried everything, soft reset, hard reset, and I even downgraded to Android 9, but the screen still the same, I had a chat with one of Samsung experts and they asked me to bring it to one of Samsung service centers.
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