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.

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