When selecting a child of an instance in the studio viewport (ie: a part under a model), studio fails to show the other children under the parent instance until you uncollapse and collapse the instances children in the explorer.
Viewing children when selecing a child of the model in the studio viewport:
Selecting an instance in a model while the model was not opened in the Explorer tends to open everything relevant to the instance to show it in the Explorer. While doing this, Roblox Studio seems to forget about showing everything else under the parent of the instance. I assume this is the logic behind the issue.
I can basically reproduce this issue 100% of the time by selecting the child of a model using alt + click. Very easily noticable if you do this on a humanoid character, but works on anything. Probably wont work if you have opened the models children in the same session previously.
I apologise for the necro post but this is the first result in Google for this so this may be helpful to some.
Restarting Roblox Studio seems to temporarily fix this, I couldn’t find any more permanent solution however, so I believe this bug is still present. I can’t really help with reproducing it though as it seems to be random for me, and Tom_atoes’ method of reproducing the issue wasn’t how it happened for me.