During past several days(weeks?), It was the problem for me not to make a notification from MiBand. It was resolved after updating the firmware to V188.8.131.52 on this morning. At least, previous firmware update happened two years before.
After this update, I’ve not changed any settings on my iPhone, So this fix may happen with this update.
Mi Band 4 was released in Sep 2019. I feel it’s great that Xiaomi provides the firmware update even the device is three years older.