Regarding the first issue: behind the scenes AtoM uses iframes to handle related entities in some of the edit forms. In this case, it looks like draft access point term iframes are not being cleared before the form is submitted. I've filed an issue here to track this, so we can hopefully address it in a future release:
Regarding the second issue:
One of our developers has let me know that this difference is because currently, the terms autocomplete in the description edit template is using MySQL, whereas the Related descriptions autocomplete is using Elasticsearch. ES, being made to support search functionality, is unsurprisingly more versatile for indexing and autocomplete results. Work could potentially be done to move this to ES, but there are tradeoffs - indexing more term fields and making them available via the information object pages requires more memory overall, increasing system requirements.
Dan Gillean, MAS, MLIS