I am having the same issue!!! I was able to link a couple of estimates and invoices but once I started to try more it started deleting the information.  I feel as though unless you link anything new it will work but you cannot do anything that is already started.

Very frustrating!  I was very excited to learn about this feature but then quickly was bummed out!

Hopefully a fix comes soon as it is apparent that others are having the same issue I am.