We are currently using a PCF2129 with the timestamp input being used for tamper detection. It is normally running with 3.3V supply for VDD with a 3.6V lithium battery as the backup supply, and is communicating using I2C. When power is removed, VDD drops with a rate of ~0.4V/ms and the battery switch over works as normal, it is running in the standard switch over mode. However on the next power up we are noticing that the TSF1 flag is set indicating an intermediate level timestamp, the TS input pin is connected to switches that are open so no timestamp should be seen.
Is there any reason for this behaviour?