During in LPM1 mode, we can not read data from FiFo buffer.
If we change the mode from LPM1 to Normal, BMA will not keep the timing.
In order to change mode from LPM1 to LPM2, we have to use Normal mode in-between.
It means there is no way to keep time using LPM1 and FIFO, if expected number of samples is higer than Fifo length.
Is my understanding is correct?
Yes, your understandings are correct for BMA280. Please switch to BMA456 that has intenral time stamp and 1KB FIFO size.