I get something similar to T108348: Allow user to create new board at Special:EnableFlow without description locally (with my External Storage patch), when blanking an existing summary. I can't reproduce it in production currently, but I'm not sure why.
It turns out in production, the blank test I did ended up in External store with length 2 (so not really blank). Maybe due to a Parsoid difference, not sure. I forget which (VE or wikitext) I submitted as locally.