My bad... figured it out.
This worked 5JmdXtN7fDwaD8ZH2a1ksEeotDaw6WKahgCCArLZmCV8RLBAiDW
Which, strangely looks exactly the same.
It turns out, somehow an invisible (unicode?) character had been introduced after the "Daw6", when I pasted it somewhere that inserted the line break. I managed to find the non-alphanumeric using rege[Suspicious link removed]
Once I removed that character... imported the $1 into coinbase.
Ahhhh..... I've seen that issue loads of times. Mostly when copy/pasting passwords, though.
Why would there even be unicode characters for '' (nothing)?