I observed this issue with nested Embeddables. That is, I have Embedded (not ElementCollection) as above.

Within this Embedded, there is again an Embedded.

In this second Embeddable, the Convert annoation has no effect.

My current workaround is to use the convert annoation one level higher:

@Embeddable
class OuterEmbeddable{
  @Convert(converter = MyConverter.class, attributeName = "theFieldToConvert")
  InnerEmeddable theInnerEmbeddable
}