https://x.com/runasand/status/2017659019251343763?s=20
The FBI was able to access Washington Post reporter Hannah Natanson's Signal messages because she used Signal on her work laptop. The laptop accepted Touch ID for authentication, meaning the agents were allowed to require her to unlock it.
I do not follow the logic here, what does that even mean? It seems very dubious. And what happens if one legitimately forgets? They just get to keep you there forever?
The 4th amendment would protect you from them seizing your phone in the first place for no good reason, but would not protect you from them seizing your phone if they believe it has evidence of a crime.
Regardless, it is not the thing that protects you (or doesn't, depending) from having to give or otherwise type in your passcode/pin/fingerprint/etc.