I’m looking for a second opinion from people who may have seen similar behaviour on macOS 26.3, especially on Intel 16” 2019 models (MacBookPro16,1).

System

• MacBook Pro 16” (2019)
• Intel i9
• AMD Radeon Pro 5500M + Intel UHD 630
• T2 chip
• Previously stable on macOS 26.2

What happened

After updating from 26.2 to 26.3:

• Normal boot results in a reboot loop.
• Safe mode boots (slow, graphical glitches).
• Reinstalling 26.3 did not fix it.
• NVRAM and SMC resets made no difference.
• Recovery sometimes inaccessible.
• Occasionally I can reach login once (often after booting safe mode first), but it crashes shortly after.

Before crashing:

• Fans ramp aggressively (“turbo-blow” sound).
• Boot often hangs around 50% progress bar before reset.

Logs / Evidence

1. Userspace watchdog panic

One of the panic logs shows:

userspace watchdog timeout: no successful checkins from remoted (2 induced crashes)
service: WindowServer ...

So watchdogd kills the system because remoted stops checking in.

Graphics stack is heavily involved (AMDRadeonX6000, IOGraphics, GPUWrangler, etc.).

This suggests a hang around WindowServer / graphics initialization.

2. SMC shutdown causes

From:

log show --predicate 'eventMessage CONTAINS "Previous shutdown cause"'
pmset -g log | grep -i ShutdownCause

I repeatedly see:

• SMC shutdown cause: -20
• SMC shutdown cause: -11
• occasional 1: Normal warm reset
• my own 3: Power button pressed

The -11 and -20 codes appear frequently around the crashes.

This makes me question whether these are hard power resets rather than pure software panics.

3. GPU / Thermal testing

Using:

sudo powermetrics --samplers smc -i 1000 -n 20

GPU die temp:

• ~62°C idle
• rising toward ~70°C
• no extreme thermal spike

So this does NOT look like classic overheating shutdown.

4. GPU switching test

I forced integrated GPU only:

sudo pmset -a gpuswitch 0

System still crashes in normal boot.

So this does not look like a simple “discrete GPU dies when active” case.

Observations

• Safe mode sometimes allows one normal boot afterwards.
• Fans ramp hard right before reset.
• Sometimes crash happens before login.
• Sometimes shortly after login.

This feels like:

• Either a power rail / logic board instability triggered by 26.3 power state changes,
• Or a graphics driver/power state interaction introduced in 26.3,
• Or something involving T2 / BridgeOS behaviour under load.

My current hypothesis

The combination of:

• watchdog panics (remoted / WindowServer),
• repeated SMC shutdown causes (-11, -20),
• no thermal overheat,
• no fix via reinstall,
• no fix via NVRAM/SMC reset,
• not resolved by forcing iGPU,

makes me suspect either:

1. Logic board / power delivery instability exposed by 26.3
2. GPU power domain issue
3. A regression in 26.3 affecting Intel + Radeon 5500M machines

Questions

• Has anyone seen similar watchdog + SMC -11/-20 behaviour specifically after 26.3?
• Has anyone with a 2019 16” Intel model experienced new boot loops after 26.3?
• Did reverting to 26.2 restore stability?
• Has anyone confirmed whether -11 / -20 are definitively hardware power faults on T2 Macs?
• Is there any known 26.3 graphics / Radeon issue on Intel models?

I’m trying to determine whether I’m looking at:

• A coincidental hardware failure,
• Or an OS-level regression that exposes borderline hardware.

Any additional technical insight would be greatly appreciated.