had mine fixed by having fingerprint reader replaced (free under warranty) at local UBreakIFix (they know all about this issue) and I was sent there by samsung support when I reported the issue
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I just got my S9 back from repair at a local UBreakIFix (Authorized Samsung Care) and the issue is fixed!! I got my S9 last week (I am not the original user) and immediately noticed the phone shutting off after going to sleep / screen timeout. Googled around and found this thread and others pointing me to fingerprint reader involvement. Reached out to Samsung support on chat, verified still under warranty. I verified for them that the issue goes away when fingerprint unlock is off. Told me I needed repair - could send it in or take to local authorized repair. Called up local UBreakIFix and they were well familiar with issue. Brought in phone, said repair wouldnt take long, simple test and likely replace of fingerprint reader, but I had to leave it over night since it was end of their day. texted me next day all fixed. Discussed repair with them and asked is this a case of fingerprint reader going bad or more of a bad batch in general. he said he thinks it was more of the latter (bad batch) - all he knows is they test the fingerprint reader and if it comes up with certain range of codes (assume code equates to certain batch or dates) they need to replace the reader. And poof all fixed. He said they first started to see the issue last August but someone in their chain eventually figured out the fingerprint reader was at involved and eventually bubbled up the info to Samsung. He said they have seen the issue on S9 & S9+ not the Note 9 but maybe he was unaware of those.
The rep I originally chatted with at Samsung didnt say they knew of this issue - to the contrary made it seem like they didnt - but once I could reproduce cleanly the issue he sent me off to my local UBreakIFix franchisee for my under warranty repair. I dunno if this can/will be fixed via software update - perhaps it could - but in meantime I can say the work done for me has resolved the issue.
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