When adding Custom Blocks the weight is changed for the rest of the blocks We are using custom blocks very extensively. We are also using block visibility groups and Domain Access. At this point when we make a new block it changes the weight of many of the other blocks at what seems to be random intervals.

Has anyone else had this issue?
If you have blocks that have the same weight, that may trigger a bug where your blocks get shifted. At least, this is something I have experienced with views filters in D7, and I found a bug report here regarding a similar weight issue in D8 with taxonomy terms: https://www.drupal.org/node/941266

Ensure you are able to see the numerical weights of the blocks, and make sure they all have distinct weights.