Block modal oddities

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Hi

What would prevent the normal behaviour of the block modal window instantly showing when a new block is added? The block adds as an empty space, but then I have to click Edit to get to its UI.

I'm overriding the block in the Manual Nav package (/packages/manual_nav/blocks/manual_nav -> /blocks/manual_nav) and pretty much all the files. The modal shows when I edit the block, and essentially works fine, but something has changed since I've edited it the overridden files.

Has anyone encountered this before? What would be the best way to track down the bug?

thanks

version 5.6.3.3