We have always used manually pinned blocks on all courses across our five sites. Today we decided to start using sticky blocks to ensure minimum layout conformity across courses and after selecting the desired sticky blocks, we deleted the duplicate pinned blocks from the courses. On returning the front page, all the html and poll blocks on the right side of the front page had disappeared. This occurred across all five moodle sites (that will teach us to not test properly!) so we can be sure that it's repeatable in our configuration.

Workaround is that we deleted all sticky blocks for courses and returned to using pinned blocks. We also had to recreate the missing front page html and poll blocks manually. Using pinned blocks is not a major inconvenience at the moment but the loss of blocks from the front page has taught us to be rather more cautious in the future. It's flagged as major because information is lost from the site and database.