TL;DR - Turning on the flashlight from the quick settings menu or lock screen shortcut runs the flashlight at a significantly lower level of brightness than is achievable with third-party apps. Another user has reported this on the Google IssueTracker with stock OS beta versions and I can confirm this on my Pixel 6 running the (as of writing) latest version of GrapheneOS (2024022800).
When browsing through the Google IssueTracker, I noticed an issue that a user had filed last December about lower flashlight brightness levels on their Pixel 6 Pro when using the lock screen or quick settings menu toggles compared to a third-party application (Flashlight-Tiramisu). I also use an app (FlashDim) to occasionally use a lower brightness level than the system toggles allow, so I decided to test it and can confirm on my Pixel 6 running the (as of writing) latest version of GrapheneOS (2024022800) that the app allows selecting a brightness level that is much higher than the system toggles. I tested it with both FlashDim and Flashlight-Tiramisu and for both apps the system toggle brightness is slightly dimmer than even the 50% brightness presets. The issue on Google IssueTracker seems to imply that this also happens on the Pixel 7 and 8 series, but I am unable to test those as I only own a Pixel 6.
I'm not sure if this is a bug or intentional behavior and how long it has existed. If anyone that reads this has the time to confirm this on their device or has more insight into the issue, please reply and also mention your device model.
The Google IssueTracker thread: https://issuetracker.google.com/issues/316690974
FlashDim source code and releases: https://github.com/cyb3rko/flashdim
Flashlight-Tiramisu source code and releases: https://github.com/polodarb/Flashlight-Tiramisu