For starters, if you hit the "advanced" toggle at the top, it shows all editable fields normally - this is okay and more or less required now to see and edit all fields correctly.
Second, without hitting the advanced toggle on a rule edit you will not be able to edit the Source Port, and the inner-menu toggles to open or hide additional Source/Destination/etc. options is inconsistent (you end up hiding Destination elements by trying to show more elements).
For now, it would appear the best method to use the editor is to click the toggle for advanced when editing a Source NAT (Destination NAT does not appear to have this issue).
Everything appears to be working well, just a GUI bug it would seem on the menu interface around editing Source NAT rules.
I hardly ever use the source port field in a source NAT rule - after all, that usually is on Outbound NAT rule that applies to an interface and most probably to all addresses that can originate from it. Thus, I do not find it unusual to hide that setting under "advanced".
The only thing I find strange with the blocks is that "Destination (advanced)" seems reverted wrt all other settings: Wenn the arrow points down, the subordinate settings should be visible, like with all other items. If it bothers you, you should open a bug on Github.
I think Source Port is intentionally hidden under "Advanced" because its meaning is commonly misunderstood, and people often erroneously set it and then wonder why their rule doesn't work.
The backwards chevron icon for "Destination (advanced)" does indeed look like a cosmetic bug.
There is something fishy with auto collapsed headers.
https://github.com/opnsense/core/issues/10601