I have the same problem with iPhone 11 Pro max. 
Dear apple I have paid 1 lakh 24 thousand rupees ($1669.04) for the iPhone 11 Pro max but the video recording issue is such that the first 10 seconds is very very low and later on the video records at normal volume. 
This issue is since last one year I wasted my money with iPhone. It was better that I would have used any local phone that would have been better than this piece of plastic.
Please apple fix this issue it is your problem your service center people treats us like inhuman and take huge amount of money and still their fix does not works.
Apple please take this issue seriously. I have taken iPhone to shoot videos but it is of no use for me now. and now my warranty is also over.

Please apple fix my Issue.
you can email me on jaiswal.dhirajkumar@gmail.com or call me Urgently on  +91 9867460266.