Just put 2 loaves multigrain in oven, Proofed over top of pan; When I checked after 30 min baking bread had fallen and was below top of pan! What happened?
It is possible to let the bread rise too high before baking.
Whole grain can suddenly rise at the end of proofing and if it does this, it is overproofed. Look for poke test in the search box.
You are better off de-gassing and rising a 3rd time, if you think there is enough oomph in the yeast.