This vulnerability stems from Bitcoin's lack of a "reverse" time limit on MPT and the fact that manipulating MPT can be profitable for miners without necessarily harming their long-term interests. Consequently, MPT fails to offer a secure, decentralized source of time, challenging the notion that Bitcoin provides a reliable foundation for decentralized timestamping.

The mechanics of Opentimestamps involve aggregating attestations to affirm the position of value, significantly reducing transaction fees. However, this system's integrity is at risk because a hashrate majority can manipulate MPT to make attestations appear earlier than they actually occurred. Such actions could potentially increase the net present value of their mining efforts without adversely affecting the current or future value of Bitcoin on exchanges.

Bitcoin's design does not enforce monotonic timestamps across all blocks, contrary to the principles outlined in Lamport's 1978 paper, which emphasizes the necessity of monotonicity for achieving distributed consensus. The absence of this feature allows a hashrate majority to delay MPT unless they control an overwhelming portion of the network's hashrate or risk orphaning blocks from honest miners. This dynamic suggests that the potential profits from manipulating attestations may not outweigh the resultant harm to Bitcoin's market value, especially as the number of orphaned blocks from honest miners increases.

In conclusion, incorporating monotonic timestamps could rectify these security issues by aligning MPT (and thereby Opentimestamps) with PoW, ensuring that timestamp claims reflect real-time occurrences accurately. Currently, Bitcoin operates under two consensus mechanisms: one based on PoW governed by individual block timestamps and another on a median of block times (MPT), which acts more like a permissioned system. This duality highlights the need for a more consistent approach to timekeeping in decentralized systems to maintain the integrity of timestamping services like Opentimestamps.