External monitors show blank screen until restart
MacBook Pro (15-inch, 2016) with 2 USB-C to HDMI adapters. The setup works great if I start up in that configuration.
If I connect the adapters while the machine is running or simply sleeping, one of the monitors will never get a signal and stay in low power mode. The OS sees the monitor, it shows in the display arrangement settings and I can move the mouse into it, but until I restart, I can't see anything on it.
Once I have restarted I can let the machine sleep or whatever, and the display still works unless I disconnect the adapter. As soon as I do that a restart is required to use it again.
Using MacOS Sierra (10.12.4)
Using this adapter: https://www.amazon.com/gp/product/B01IUXA9YK/
The video signal on the Thunderbolt connection on your MBP is actually Display Port so you are actually going from Display Port to HDMI. I am not a fan of converting signals because it's flaky at best.
If you must connect to HDMI, try an active1 adapter like the TRENDnet USB-C to HDMI 4K. They have two versions - one that does 30Hz and another that does 60Hz (it's the Power Delivery version).
What is preferable, is to go from USB-C to Display Port which is actually Display Port to Display Port, so no there's conversion involved. No conversion means less points of failure.
jksoegaard's comments provides more evidence of this:
Just to add a real-life experience: I'm using 2 USB-C to DisplayPort adapters (which are basically similar to the HDMI adapters) - and this works without any problems in that regard. Several times a week I plugin in both monitors after the MacBook Pro is booted (basically I never turn it off) - removing the cables again also never give problems
1 I wrote an answer on this very topic: Passive vs. Active Adapter
Update to latest OSX version (10.12.5 (16F73)) fixed the problem.
To the above answers, I can't use a USB-C to HDMI active adapter as Macbook pro provides only two USB-C ports. I don't know how a company like Apple comes up with a crappy design like this. I use 2x of this adapter: https://www.amazon.com/gp/product/B07NDH6DRN/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
1 HDMI works perfectly. 1 HDMI needs a restart every time to work (exactly the same problem as described in the question).
I'm running on the latest OS version: 10.14.5