Here's the last tune I leaned by earI’d bet if both of you (@cappers and @mizmar) .... you’d be able to play along with it an hour or two after that.

Theory & Impro - Improvising and all that jazz
YouTube - with the arrangements on the site, under the YouTube tab View: https://youtu.be/SfNe1inUi7E Yes I found it! I have been mainly focussed on the lesson sections and not really delved into the You Tube bit. I am currently working through the Learn to Improvise section and Music...

And, sure, it's a tremendously important and useful thing; and I practice it regularly..
... But, wrt to my OP ...
Before that, the previous tune I learned by ear was for my flute teacher.. a tune he really loves and has recorded, and I'd check myself with a metronom and recorded backing (he'd prepared) ... And he still wasn't happy. And it wasn't the couple of technical errors (one triplet too slow; one f-half sharp to high); it was overall.
The only thing I can think of is that I was ambivalent about the tune. I wasn't playing music, I was completing an exercise.
And it's not just that context, I'm the same on sax (often) or, now, singing...