There are always exceptions to everything…
-I actually have done WFH where I created music specifically for a production under a WFH agreement and not only kept my writers share but was also granted 50 % of the publishers share.
-I have done a ton of WFH and have always kept the whole writers share though there have been times I have been asked to relinquish it or a portion of it.
-As for the difference in my experience of 46 years working in the biz… usually WFH is for new music created specifically for a production, library, etc. during a term and an exclusive perpetuity agreement is usually for existing works taken on and over by a company on an exclusive basis. I have always retained the writers share for those as well and sometimes some of the publishers share.
Every contract can be different.