Honestly I don't see how the presentation cache could be shared is it makes OBIEE unusable. The presentation cache is personal, it's like browser cache in the end.

When you see the wrong data with the user, what does the log tell you? The LSQL must be executed at least (the presentation cache doesn't even execute the LSQL anymore, it just show you the cached entry).

It looks more like you have an issue on your VPD implementation and the query is executed on the connection of the previous user instead of working as VPD is supposed to be on an isolated new connection.

Logs have all these infos inside, look at https://www.rittmanmead.com/blog/2015/03/instrumenting-obiee-database-connections-for-improved-performance-diagnostics/ on how to be able to track things down to the DB side and analyze all the steps.

I would definitely bet on a connection pool / VPD issue and not on presentation cache.