I didn't feel like I did something specially weird.
Patched both DLC 1 and 4, installed 60 FPS mod & NoMonastery 1.2.7.
Picked New Game / Casual / Hard, chose the Black Eagle route, did the prologue and first few maps no issues until the first map with fog, where all enemies got +99 stats for some reason.
Made a save, quit the game, rolled back NoMonastery to 1.2.6 where the enemies didn't have the buff. Once the map was done, I saved, quit the game, went back on 1.2.7, then proceeded with the game, killed the Death Knight then the boss on chap4, did the chap5 map. I let Gilbert die but proceed with finishing the map. I get the save, the "Rumors of a Reaper" title card, then dialogue with Rhea/Edelgard, get a save prompt, then get sent on the calendar for the 9th month, where it loops.
I had a save before chap5 map, so I tried completing it again, but it ended up with the same result.
Maybe I did something wrong, I don't actually know. Something that's slightly bothering me too (as in I hope I didn't mess something up) is that everything character has their post timeskip portraits. I just assumed it was intended seeing the mod youtube video.