Sorry for responding late, there is an option in the mod to reduce/remove artefacts, you can try enabling that.
Thanks, good news, bad news Chrysalis.
Bad news is if you meant the setting called "Battle menu artifacting fix", I tried it that time and there's no change. After playing around with the settings, seems like it's not only happening to the bars but everywhere, didn't notice it because I used the original border size but when I use Thinnest UI border, I see it everywhere.
The good news is I think I may have accidentally fixed the issue ...for now
Apparently I went and play around with FFNx settings and found out that if I set the internal_resolution_scale to 2, the artifact disappears
So I'm sorry for this oversight, I'm a noob when it comes to these kinds of settings, I didn't know that I have to do that when using 16:9 ratio