Incompatible with Preference.h (Flash writing)

[NilsIoT]

On a Firebeetle ESP32 board, Servo.h can make it impossible to write to Onboard Flash. Reproduce like this:

1. Attach more than 8 servos (I used 9) (in software; hardware doesn't matter)
2. Sometimes required, sometimes not: Output PWM on 9th attached servo using any pin. It is not important what you do with the first 8 servos or if you use them at all.
3. Try to use Preferences.h to write to on-board Flash: Cannot write, and read only returns the default value. No error messages.
4. Detach Servos
5. Try to use Preferences.h again: Works fine

The error occurs irrespective of what pins are used for the Servo. I suspect a problem with timer allocation or something like that, but I am no expert in ESP32 timers or the two libraries involved.