Provider produced an unexpected new value for was present, but now absent...?!

Can somebody explain to me what this error means, please?

I’ve built my own terraform provider which creates an Avere storage cluster on AWS, and am looking to open-source it once it works properly… and it does work, but after running successfully and building the cluster, it always returns with:

Provider produced inconsistent result after apply: When applying changes to module.avere_caching.avere_cluster.test_cluster, provider "avere" produced an unexpected new value for was present, but now absent.

Would really appreciate some idea of how to debug this…