What safety or application-level bugs could arise if developers assume Snapshot Isolation but Amazon RDS for PostgreSQL is actually providing only Parallel Snapshot Isolation, especially in multi-AZ configurations using the read replica endpoint?
Consider a "git push"-like flow: begin a transaction, read the current state, check that it matches the expected, write the new state, commit (with a new state hash). In some unfortunate situations, you'll have a commit hash that doesn't match any valid state.
And the mere fact that it's hard to reason about these things means that it's hard to avoid problems. Hence, the easiest solution is likely "it may be possible to recover Snapshot Isolation by only using the writer endpoint", for anything where write is anyhow conditional on a read.
Although I'm surprised the "only using the writer endpoint" method wasn't tested, especially in availability loss situations.
