MBP Folder/Question mark icon, but boots ok with Option key
It may be worth you resetting your NVRAM.

Resetting the NVRAM

Here’s how to reset the NVRAM on your MBP:


Shut down your machine. Yes, a full shutdown, not just logging out.
Press the power button and then press the 
command option P R keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down until your Mac reboots again (i.e. a 2nd time) and you’ve heard the startup chime twice.
Let go of the keys and let your MBP reboot normally.


Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc). As a minimum I would check your Startup Disk settings.