Sudden leaf yellowing/browning on two different vandas

[UK Orchid lover]

Hello,

I have been growing vandas for about 10 years and suddenly two of my vandas had a single leaf near the bottom start to go yellowy and brownish in the middle/end of the leaf which I have since removed and then sealed the cut.

The temperatures have dropped recently because of autumn, it looks like some kind of infection to me but I am not sure. The rest of the plants look completely fine except the one leaf on each that gained this issue.

Does anyone have any thoughts on what and why this occurred? There does not appear to be any bugs after close inspection but please let me know if you have experience with this.

I don't believe the dark parts are sunburn because it ends in a straight/diagonal line with purple tint on the border.

(the mess on the paper is excess cinnamon)

Thank you!

[estación seca]

How cold was it in the room? Most people advise not letting them get below 55 F / 15C.

How old is that leaf? It could be a very old leaf dying naturally.