Leaves are turning yellow during flowering - is this an overwaring issue?

If its yellowing from the buttom upwards its not enough nitrogen If its yellowing from the top downwards its too much nutrients and or too much light

“ Given that I don't use fertilizers, there are no big changes in temperature ” First off - not sure what this has to do with temp change?? More importantly. Here is your issue. Assuming you used an actual soil with nutrients, you were good on that for a few weeks but soil alone will not sustain a canna plant thru harvest without supplementing nutrients. It’s like a baby being on milk from its mother for 18 months, stopping feeding in that manner and than asking why it’s looking deathly after a week of only giving it water. Crude example but hopefully you understand. You must use fertilizers/nutrients to sustain the life.

I don't see any nute information in your diary for this grow... are you not feeding her? She's absolutely starving for some good NPK - more P and K than N... when they get into flower, they are gobbling up almost as much P and K as you can give them - they don't have much use for N but they still do need some... And no, this is in no way an overwatering issue - in fact, the further they get into flower, the more water they're going to need/want ... all this lady needs is some good, old fashioned FOOD... and btw, the yellowed leaves are not going to ever get green again no matter what you do but once she gets proper nutrients, the spread of the yellowing will cease. You should also be giving her a dose of calmag once a week as well... Good luck!

Most likely a nutrient deficiency. I would get a bloom fertilizer asap.