I think this happened exactly once for me with this Hoshino manga we don't talk about here. I actually removed my rating from it.
If an ending is that bad while the rest is so good I could not recommend it to anyone, but I could also not say to not read it...
So if there isn't a possible rating I could give it, it's just not getting one.
And I almost feel sick even thinking about it so I'll stop here.

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in need of romance?
uncommon, sad, sweet

Remembering Tokyo ghoul re... Started great after the prequel, got meh and then had a ending that felt like a slap in the face.
So, about 7 to 8, really depends on when the decay set in.

Usually a series drops in quality prior to a really bad ending in my experience, so chances are high I dropped the manga before seeing an ending I actually hate.

Depends.
Most manga that have a crap ending don't have a crap ending that retroactively ruins the entire comic. there are a few cases where it does in fact 100% ruin it though, in cases where it doesn't, 6-8 mostly falling in 7 in cases where it does, I wish I had brain bleach.

If it was axed then I ignore the ending completely. For other it depends how much it dragged and changed the rest. If it's just the ending it might be an 8 or 9 but if it maked me change my opinion of other parts of the series it might be a 5 or 6