looks like you found a bug in WP 5.8.
Its not a GenerateBlocks issue – it seems trying to drag multiple elements inside any nestable block eg. Group Block, Cover Block throws the same error.
I tested this on the core TwentyTwenty theme just using the core blocks. And it throws an error.
Yep… as we’re talking about this, do you know if it’s a well known bug with Gutenberg that when you try to select various words in a text line using SHIFT key it’s freezing everything and you can’t edit any block ? You have to refresh your page or delete block and do a CTRL+Z to be able to edit text again…
Its not something i have experienced or neither have i seen other users report it.
If you can replicate the issue, and explain how to, then the best bet is to raise it on the Gutenberg GitHub.
There continually looking to improve the performance of the editor, so if its a known issue i would expect it to be on their list of fixes.