Adult trauma doesn't do this because the foundation has already been properly laid
I will disagree with you about this. In actual fact, based on resilience studies, children have far better resilience than adults. It is often in adulthood where trauma happens, and foundations crack. It doesn't matter whether your a child or adult, the same underlying foundation cracks. There are always caveats to either side, but that is the majority based on studied result data.
Example, for many CSA trauma, the child moves through childhood and into adulthood, fairly normal. There may be behavioural aspects as a result, absolutely. Otherwise, a functional adult. Something happens in adulthood, resilience is not as good as childhood, suddenly a crack is formed, then all the memories of previous trauma add weight, further cracking the foundation, and an adult suddenly crumbles.
Their foundation was solid, but once a crack formed in adulthood, the additional weight caused more, opening every wound.
It happens so readily even in adulthood after healing. Endure another trauma, and people will tell you how everything is now a problem again, even though they healed it and moved on with life. The brain is tricky, and PTSD trickier. PTSD will literally attempt to use every weapon at its disposal against you once resilience collapses again. Your foundation can be solid for years, decades, then it grabs hold of a recent event, grabs everything in your past and throws it back at you, even though you already dealt with it all prior.
PTSD will make you feel back at square one, even though you aren't.