  In response to the bottleneck of processing throughput inherent to single
chain PoW blockchains, several proposals have substituted a single chain for
Directed Acyclic Graphs (DAGs). In this work, we investigate two notable
DAG-oriented designs. We focus on PHANTOM (and its optimization GHOSTDAG),
which proposes a custom transaction selection strategy that enables to increase
the throughput of the network. However, the related work lacks a thorough
investigation of corner cases that deviate from the protocol in terms of
transaction selection strategy. Therefore, we build a custom simulator that
extends open source simulation tools to support multiple chains and enables us
to investigate such corner cases. Our experiments show that malicious actors
who diverge from the proposed transaction selection strategy make more profit
as compared to honest miners. Moreover, they have a detrimental effect on the
processing throughput of the PHANTOM (and GHOSTDAG) due to same transactions
being included in more than one block of different chains. Finally, we show
that multiple miners not following the transaction selection strategy are
incentivized to create a shared mining pool instead of mining independently,
which has a negative impact on decentralization.
