Monthly free games same as ones received as PS4 collection
Solution 1:
According to the official Playstation webpage:
The PlayStation Plus Collection
Redeem and play a curated library of PS4 games that defined the generations. Access the entire collection on your PS5.
Emphasis mine.
Additionally, this YouTube video from the official PlayStation channel introduces the PlayStation Plus Collection as "a new benefit for PS5":
So the PlayStation Plus Collection is only for PS5 owners. Meanwhile, this month's Mortal Combat X is for PS4 owners.