I can confirm the issue since Drupal 10.0. I'm using an XYBlockBase class inheriting BlockBase in my module, that lazy-loads the block body using the big-pipe technology. This class is extensively used in my module, mainly to load lists of rendered entities with content sizes from 10k to more than 1M. Since I've updated my Drupal instance to version 10 last week, all these blocks didn't load its content anymore due to the above described issue of an incomplete script-node, when the MutationObserver is triggered.
Side note: The upgrade to Drupal 10 worked on my local development environment for smaller content (<40k), but failed on the stage server. So the streamed size of the script-node content, when the MutationsObserver is triggered, depends on the runtime environment.
The patch #2 solves this issue for all lazy-loading blocks. The solution of patch #2 is clean and not invasive, because all non-JSON-parsable script node processing is just postponed until the document loading is complete (DOMContentLoaded event).
PS: The references to the non-parsable script nodes stay the same, because only the script-node content is updated by the HTTP streaming of large placeholders.