See https://bugs.gentoo.org/795813
The issue is on the second line of xcas.desktop. The "Version" field is not the version of giac/xcas but the version of the .desktop file specification. In this case it should be 1.0 instead of 1.7.
This is still present in 1.7.0-13 which is the latest version available at the time of writing.
giac .desktop file is incorrect
Modérateur : xcasadmin
Re: giac .desktop file is incorrect
Fixed for next version, thanks!