Honestly, the reason why this ending is unsatisfactory to me isn't because we don't know how they got out of the loop but because the axe made the story rush what made it unique.
To me, this story was never explicitly about the loop itself. Sure I was curious about how she initially got into the loop, how he was able to join her in it etc., but all of that took a backseat to the "mundane" content we got. A peer into their bleak, repeated days. The story was predicated on their feelings/growth (or rather regression) within the time loop. And there was something so compelling about the two of them being stuck together and not really trying to work out why the loop is a thing, investigative style (as most loop stories do), but rather focus on trying to feel and experience as much as possible despite their situation.
The degeneration of their mental state was what made this so good, and to not be able to see it carried out to the very end, to see what happens after Mashiro "got what she wanted", to see why they supposedly drifted apart... and instead having to see it as some film strip panels we have to guess from, that's what makes this sucky for me.
If we saw everything they endured up until the breaking of the loop, I don't think an unexplained breaking would've been that bad (though I'd still have some complaints), and it would've definitely made the ending a lot more emotional to see them go through so much only to thankfully not remember any of it.