yes I've wasted too much time on this already.
I love a mystery so I took a copy of the faulty version & tried deleting stuff bit by bit & dropping it back into ADE e.g. removed cover, notes, index... eventually it worked but when I started over & tried to replicate that, I failed.
I saw that with the fails, ADE could not produce a sensible thumbnail, but as I say, I cannot replicate the issue reliably so time to move on I think.
At some point I opened & closed the sigil metadata editor but repeating that action alone was insufficient.
The calibre epub to epub convert is a good alternative to banging your head against the wall - it seems to fix stuff that otherwise is not obvious.
I am inclined now to NOT blame the modify epub plugin - I think user error i.e. me, is a far more likely culprit
