(updated topology screenshot to match the question)

I'm testing the following topology:

Single master that has semi-sync replication enabled, with single semi-sync slave running in another DC. The idea here is to gracefully failover to another DC in case master's DC goes offline.

I have a PostFailover hook that makes sure that, in case of master failover, semi-sync replication is enabled on either replica from another DC (preferably) or replica in the same DC as master and disabled on all other replicas.

This seems to be working fine, however I can't figure out how to handle failure of semi-sync replica - there is no hook executed when replica is lost, and so master stops accepting writes as it's waiting for ACK from its remaining replicas (none of which have semi-sync replication enabled).

Is this somehow related to Semi-sync enforcement? I've already raised similar question on the ML, but it's not obvious from documentation and code that this feature is a correct approach here.

One idea I had was to turn that topology into something similar to that:

In this topology semi-sync replica is also intermediate master, which triggers hooks on failure and lets me reconfigure other replica for semi-sync replication.