Old backfill data gets deleted even after saving

I have seen this unusual behavior multiple times. Here is whats happening.

I have Upstox API subscription to backfill data.
In one go I backfill 50 symbols. Then say next day i backfill another 50 symbols.

When i return to other old backfilled symbols, some of the symbols have no data. I again backfill to make sure there was no error. But next time i check data again some or other symbols are missing data.

Note that I always save database before exit.

Please let me know how to solve this issue as i don't want to lose data over time.