If you see a "Discovery Mode" line with a negative value in your Curve report, that's correct and part of how it normally works.
The negative value represents the 30% commission that Spotify withholds only on streams generated through Discovery Mode, shown directly as a deduction.
Keep in mind that the report only breaks out the deduction (the line with subsource "Discovery Mode"). The earnings from those streams aren't shown separately: they're included together with the rest of your Spotify earnings.
What is Discovery Mode
It's Spotify's program that boosts your tracks in algorithmic contexts (like radio and autoplay). Spotify applies a 30% discount on the streams generated within those Discovery Mode contexts. Streams that happen outside the program are paid normally. That 30% on those streams is what you see reflected in the negative line.