First recommendation: When adding assignees to an issue, the button “Clear all assignees” appears at the top of the list of assignees. This makes it really easy to inadvertently click on that – and it’s been happening often enough to warrant spending time suggesting it here. Moving it to the gear menu would be better, since it’s (presumably) a “rare” use case, vs. adding new assignees.
A second recommendation is to buffer changes to issues until a rapid successions of changes are completed. This prevents spam such as coming from above when all assignees are inadvertently removed and re-added within a short period of time. 10 seconds? Keeps the issues tidier, particularly when adding a label, an estimate, a sprint, a few milestones, and releases at the beginning of an issue.