Possibly, because he really didn't want to write a letter at all, but he wanted to do it anyway for you, because it would be awkward to say no I won't. It reminds me of my spouse, who would rarely outright refuse a request from me. Instead, he'd just do a lousy job, because he wanted to be the kind of guy who "helped" around the house and because he loved me and wanted me to be happy. And it wasn't my exacting standards of what you had to do to please me, it was, as he admitted, a mostly unconscious message to stop asking him to do this thing.

Edited to add: I think even under the best of circumstances, a letter and sometimes even an email or anything written is a hard exercise for T's. My T said one reason why he likes his job is because it uses his verbal and listening skills and doesn't require communication (except what he considers rote stuff for insurance) by writing. He doesn't like to write and doubts his ability to communicate effectively in that mode, hence why he dropped out of his creative writing major. They also don't get training on written communications and I believe from reading stories here, that at least some T's really suck at it. But I think, more broadly, that you may be reframing the "evidence" to support your "old self" position that he doesn't really care. Maybe it is the work of the confirmation bias: Confirmation bias - Wikipedia