You’re currently using the sans-serif math characters of the default “New Computer Modern Math” font. These sans-serif characters only exist for a limited set symbols, and for some reason, Unicode decided that regular (non-bold) greek is not part of that. You can find a list of all the different math character styles supported by unicode here.
However, you can use the “New Computer Modern Sans Math” font, which doesn’t require you to call math.sans anymore, as all characters are already sans-serif by default. Since you can then just use the regular greek letters, the fact that non-bold sans-serif greek math characters don’t exist, won’t bother you anymore. The font is not available on the web-app by default, but you can get it from CTAN (as NewCMSansMath-Regular.otf).
// Default font with sans-serif character set
#show math.equation: set text(font: "New Computer Modern Math")
$ sans(alpha = cos^(-1) (8 / (sqrt(29) dot 4)) approx 68.2 degree) $
// Sans-serif font with default character set
#show math.equation: set text(font: "NewComputerModernSansMath")
$ alpha = cos^(-1) (8 / (sqrt(29) dot 4)) approx 68.2 degree $
