In UI Builder sometimes all of a given object's children won't show up in the list until you force refresh

1. Install Unity 2023.3.0b3 for scenario week
2. Install UI Toolkit and use UI Builder to try and make a custom slider
3. Note functionality:

Bug:  There is an issue with the hierarchy displayed in UI Builder that would sometimes cause some or all of a given object's children to simply not show up in the list until you forced it to refresh somehow

Note: we only had one report of this in the channel - not sure how prevalent it is https://unity.slack.com/archives/C06CT8REG6T/p1705267355939019?thread_ts=1705264628.158019&cid=C06CT8REG6T