was the car recent driven over poor roads?
if you operate the brake while stationary do you see more bubbles?
had the car completed several heavy braking events (to test brakes etc) in quick succession causing the brake fluid to boil?
Because of COVID I am hardly driving, but this is the first time I ever checked the brake fluid reservoir. Prior to checking it, I moved my car to wash it yesterday (4-4-20) so it did not get time to get any type of heat and prior to that I drove it on the 3-4-2020 for less than 30 mins early morning. I have had my vehicle since 2017 and have never experienced any brakes issues.