I did mean that the threshold should be zero.
Did these images originate in Gimp 2.6 ?
I can not find the reason behind the difference from your xcf files but the fix is the layer mode.
Set the mode to legacy - the right side icon
Can you post the original 2.6 image before opening in Gimp 2.10 ?
If you are using Windows then there are Gimp 2.6 installers https://download.gimp.org/gimp/v2.6/windows/ and these do co-exist with Gimp 2.10
screenshot Win10 / Gimp 2.6.8 (64 bit) https://i.imgur.com/3VvTAco.jpg
Did these images originate in Gimp 2.6 ?
I can not find the reason behind the difference from your xcf files but the fix is the layer mode.
Set the mode to legacy - the right side icon
Can you post the original 2.6 image before opening in Gimp 2.10 ?
If you are using Windows then there are Gimp 2.6 installers https://download.gimp.org/gimp/v2.6/windows/ and these do co-exist with Gimp 2.10
screenshot Win10 / Gimp 2.6.8 (64 bit) https://i.imgur.com/3VvTAco.jpg