I was searching the forum if this was discussed already somewhere and found this thread (from 2017) by @bjoern:
Actually I don’t find it very useful and very annoying that regions snap back to the minimum size of its content and always thought it is a bug. @gregsn already suggested two possible solutions back then:
A) after manually resizing the region is in a pinned state. Releasing the pin brings back the auto compact feature.
B) after manually resizing the region is in a pinned state. There is no pin that you need to navigate to as this can be painful in larger regions. Shaking an element inside the region brings back the auto compact feature.
I would totally, like 100%, wholeheartedly vote for option B! Resized regions should stay resized!