nute burn, or something else? and what do i do from here?

My girl is going into 6th week of flower, and a few days ago I started to notice evidence of nute burn. Symptoms are pretty classic, except its only affecting the topmost leaves on the plant - right around the main cola. Bottom leaves look fine. From reading around on other posts here, it seems like this would typically start on the bottom leaves first. Any reason why it would start from the top like this? Or am I'm dealing with another problem entirely?

Additional info: Growing in coco coir in a clay pot - feeding Canna Coco A&B at full strength (but just plain water every other feeding) - PH levels have been steady.

Any help much appreciated - thanks guys!

What's your PH?
The light color looks like N def., and the rust colored spots remind me of the Mag def. I had on my Sativa.
Hopefully the charts will help, if you don't get any other replies.View attachment 2391339View attachment 2391340

Thanks for the reply! I hadn't considered a mag deficiency - my mind went directly to nute burn when i saw the leaf tips curling down, but looking at the ph chart that would make sense, as my PH stays closer to 5.5-6.0 range. I'll try to correct that when I feed tomorrow and see what happens.
Any thoughts on why this would start in the top leaves, rather than from the bottom as usual? That's what really threw me for a loop.

You're right, usually it starts low, that's why I'm not sure, but my Mag deficiency started randomly.