As someone said years ago earlier in the thread: it does look like sun-scorch. Is that possible from the position they've been in (i.e. direct sun)? I can imagine the shapes on your leaves being caused by where shadows were cast by other leaves.
It is getting the time of year when plants suddenly get exposed to direct sun at low angles, which they won't have seen since last year. Even my cacti can suffer in the greenhouse. I've no idea whether Canna are particularly susceptible, but I've sometimes managed to scorch similar marks onto Strelitzia, whose foliage looks similar.
@danieldrennenEatRTq0X Just spotted this post so sorry for the delay in replying. I never did find out what caused it. All I can say is that the Canna was fine and the discolouration didn't cause any problems.
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