The reported issue: #5674
Was NOT solved.
First, you affirm that is the intended behaviour of a BelongsTo field is to retain an invalid value even though the options have changed, offering no explanation as to why or what is the possible use case for this.
Second, you offer a "solution" which I test and let you know that isn't working.
Your answer? Closing the issue as "not planned".
What's not planned? Fixing a bug?
Or giving a solution to a real, demostrable problem to a paying customer?
Frankly I'm appalled by the customer support of this closed source, commercial product.