I had some problems with attribute data not being cache synced between servers. That seems consistent with the behaviour you see - the other servers continue to use old cached data until the cache timeout is reached rather than refreshing the cache immediately when the attribute is changed.
You don't state the versions you're using, but I was using Relate 2 R2 (4.1.x.x assembly versions of EPiServer.Community dll:s) and for that version there are some known issues with cache sync. Contact support and ask for a hotfix. I received hotfix #73546 and from what I can recall it solved my problems. That hotfix needn't necessarily match your versions (and other hotfixes you have) but it could help the support engineers locate the related bug fixes.
I have a Relate+ site in a multi server environment that uses a single database. We have some functionality around topics where replies can be deleted but in reality a reply has an attribute set to "hide" the reply rather than delete it. On binding the replies we check for this attribute and remove any from the list of replies that have it set.
It seems on the multi server environment that this caches the replies so that the "deleted" entries are not hidden straight away but later they are (I don't see this in the dev environment, single instance).
Firstly, has anyone encountered this before? Secondly, does anyone know how to force a refresh of the cache once setting the attribute value of the reply (at the reply or the topic level)?
Any thoughts greatly appreciated as always,