Glad you pointed this out. For so long content creators said to avoid this portion of the shop but I been splitting like crazy for awhile using this method and it’s helped a lot with my CL.
Ive been doing this for a long time, and its been amazing. For your mysterio, it didn't have the green graphic to upgrade because you had the favourites filter on and your favourite mysterio was the custom card, which can't be upgraded.
Before they updated the bonus booster section, I found it annoying. So i upgraded every card until everything was either infinite or uncommon and then made sure i had at least 10 boosters on everything and it made the bonus booster section go away.