Hello lcdlcdlcd and welcome to the Apple Support Community.
The fact that the first Thunderbolt Display has very slow brightness control while the second one has no usable brightness control, especially after sleep, makes it useful to isolate the displays from the daisy chain before assuming either display has failed.
Your 16-inch 2021 MacBook Pro uses either an M1 Pro or M1 Max chip, and both support at least two external displays, so two displays are within the supported display count.
→ Test each Thunderbolt Display separately:
• Disconnect the daisy chain.
• Connect only one Thunderbolt Display directly to a Thunderbolt port on the Mac using the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
• Open System Settings > Displays, select that display, test its Brightness slider, then put the Mac to sleep and wake it again.
• Repeat exactly the same test with the other display.
If both displays work normally when connected individually, that would strongly isolate the problem to the daisy-chained connection rather than to either LCD panel.
→ Then swap their positions in the chain:
• Put the display that is currently last first in the chain.
• Put the current middle display second.
If the brightness problem follows the second position, the chain or Thunderbolt connection becomes the main suspect. If the problem follows the same physical display, its cable or the display itself needs closer investigation.
→ If a display is not being detected correctly after sleep:
• Disconnect it from both the Mac and power, then reconnect it.
• Open System Settings > Displays, hold the Option key and use Detect Displays if it appears.
If you have two Apple Thunderbolt 3 to Thunderbolt 2 adapters available and both displays work correctly individually, also test connecting each display directly to a separate Thunderbolt port on the Mac. That bypasses the daisy chain completely.
Please let us know what happens when each display is tested directly, and whether the problem follows the physical display or its position in the chain.
🙏
Have a good day.
And one more thing…
Never forget to think different.
Important:
• Always make sure that your Apple devices have the latest available version of their operating system and that your apps have the latest available version so you can benefit from fixes and compatibility improvements.
• Your Mac is currently listed as running macOS 26.5. The latest public version of macOS Tahoe currently verified is macOS 26.6.2. You can check for it in System Settings > General > Software Update.
Useful links:
About the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter – Apple Support
If your external display is dark or low resolution – Apple Support
Connect displays to your Mac – Apple Support
