Since around 7.0 I have noticed on SAMD21 boards that sometimes the autoreload doesn't not happen.
It is hard to reproduce and seems to happen randomly. But once the boards stops running autoreload:

• saving files works, but does not trigger autoreload
• ctrl-C ctrl-D works, but does not bring back autoreload
  Only a hard reset seems to bring it back.

I have seen the board enter that bad state:

• as soon as I plugged it in.
• after doing some file operations from the host side.
• after leaving the board plugged in without using it for an hour.

I have done some of those tests on a freshly-flashed, latest-main, and erased-file-system CircuitPlayground Express with a simple neopixel animation code.