On my computer, the component doesn’t get overwritten if I left the 2021 format.
Perhaps it’s due to the fact that it is used by Lumion and that it gets a ‘kind-of read-only’ status flag?
Anyway, I don’t get why it works when saved as 2018 skp format…
Save as 2021 skp seems to work (no error popup) but when I check the date stamp, I realize it didn’t overwrite the file.
BTW I use SU 2018 for all my export jobs as 2021 can’t export separated elements as FBX (nor DAE nor OBJ) which is a PITA for people working with other softs and who need exporting their geometry.