2019 MacBook screen fades, shows bars after Sequoia 15.8.1 update.

Because the screen fading and translucent vertical bars began after installing macOS Sequoia 15.8.1, but do not occur in Safe Mode or a newly created user account, I would investigate software or settings specific to your original account before assuming a hardware fault.

The timing suggests a possible software issue, although it does not establish that the update caused it.

Since you have already removed login items and disabled unnecessary extensions, EtreCheck would be a useful next diagnostic step.

It generates a system report that can help identify third-party background processes, extensions, startup components, system modifications, and other potential software conflicts.

Download EtreCheck from its official website and run it while logged into the affected account with the Mac started normally.

Review any reported major issues and third-party system modifications. EtreCheck can help identify potential causes, but it cannot directly confirm that a particular item is responsible for the display symptoms or rule out an operating-system regression. Review the report for personal information before sharing it.

I would also recommend these checks:

1. Compare the displays, if possible. Connect an external monitor and see whether the fading and vertical bars appear on both displays. If the external monitor remains normal, that is useful diagnostic evidence, although it does not conclusively identify the cause.
2. Check for a newer macOS Sequoia update. Open System Settings → General → Software Update. Back up your Mac before installing any update. A later update may resolve a software regression, but there is no confirmation here that a specific update fixes this particular symptom.
3. Consider reinstalling macOS Sequoia if the problem persists. On your Intel-based 2019 MacBook Pro, restart and hold Command-R to enter macOS Recovery, then select the macOS reinstallation option. Back up your data first. Reinstallation normally preserves personal files, but a backup is important. See Apple's macOS reinstallation instructions.
4. Run Apple Diagnostics if necessary. Although the symptoms disappearing in Safe Mode are encouraging, they do not completely rule out a hardware problem. See Apple Diagnostics instructions.

I would run EtreCheck before reinstalling macOS, as it may reveal a software conflict and help avoid a more disruptive troubleshooting step. If the report does not identify anything relevant, that does not rule out a macOS graphics or display-related regression.

The most useful additional detail would be whether the fading and vertical bars also appear on an external monitor or are limited to the MacBook's built-in display.