I am having the exact same issue. I bought Blackview BV9900 Pro last week and I am having this same weird problem since day one. It will happen at random times of the day, once or twice each day. Same as described by StGabriel - the phone appears to become unresponsive, it will not wake up when I press the power button on the side, nor if I try to use the finger print sensor. While the phone is in that frozen state its screen is dimly light but nothing appears on screen (just some dim light). To recover from this condition I have to hold continuously the power button until the phone restarts. I emailed Blackview support asking for help.
Sorry for causing this kind of trouble, I have feedback on the problems you encountered to our technical staff, we are already analyzing the plan and will propose related solutions, please wait patiently. If you have other questions, you can send an email to our professional after-sales staff： firstname.lastname@example.org