(More) questions about mat leave, after manager's reaction was...unexpected

I ended up letting my manager know about my upcoming mat leave, and it ... didn’t go great.

It turned into a bit of a negotiation on how long I would be gone. I did not expect that, and I felt it was inappropriate. He raised operational requirements a few times, but I thought the length of my leave was up to me only. Am I off-base there?

He also mentioned if I took more than 12 months, he could priority list me, and fill my position indeterminately. This was a surprise to me. I know this is true for LWOP, and that a mat leave is technically an unpaid leave, but I don’t think that would apply here. Could he really do that?

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EDIT: Thank you everyone for your thoughts! There is some really sage advice in here, and the sense of community really does make me feel better.

FWIW, I think this was a knee-jerk reaction from someone burned out and drowning in work, rather than malice. That doesn’t make it OK, but I’ll give it another chance before escalating. I think a firm email indicating the number of months I am taking, and quietly shopping my CV (so I can GTFO before my leave is over) is probably the way to go. If he keeps pushing, union time.