I think your ble 33 loses bootloader address.
If you have J-link edu, you can reload the bootloader.
I also experienced the same bug.
In my case, I reloaded the bootloader, then after the ble re-works well.
I think the 33 BLE works unstable.
When I use the Mega or Uno, I don't have experienced many problems.
but, now.. I need the BLE 5.0 communication,
so I'm doing a coding of the 33 BLE . but it doesn't work well T.T...
You don't know how many I reload the bootloader of the 33 BLE....
when I use some headers (wire.h, RTClib.h, SPI.h etc.), sometimes, the 33 BLE doesn't work.
Good Luck...