Unstable timestamp for pending transactions

[komret]

I noticed this when implementing trezor/trezor-suite#12975.

Pending transactions have a timestamp (first seen), but this timestamp keeps changing over time. See these screenshots of the same transaction in block explorer taken today:

Morning:

Afternoon:

In fact, this transaction was broadcast sometime in May.

[martinboehm]

Unfortunately, as of now, Blockbook is not storing mempool transactions between restarts. Therefore, the first seen time is reset by restart of the service (this is not very frequent but happens from time to time due to maintenance reasons).