For the set, it was because i use a lot of cover from dvd/bluray boxset with their own design, without the blue (red here) border. (here an example for the set Mad Max : )
HTPC - Nvidia Shield TV
Desktop - Homemade : 11th Gen Intel® Core i5-11600K @ 3.90GHz | 32GB | AMD Radeon RX 6600 XT | SSD 500Go + 750Go + HDD 500Go | --W10--
NAS - Synology DS414 : [2*8To WD Red Plus (Raid 1)] + [2*6To WD Red (Raid 1)]
KODI 19.3