PLEASE HELP! I'm going crazy trying to fix this problem with phal bellina and gigante

[Narissa44]

Hi again. So I posted a bit ago about a gigantic alba and asking what the problem is and someone said maybe cold damage. But it spread a bit and I don't think cold damage spreads? So I cut off the affected part of that leaf and so far so good with that. The thing is, something similar happened to my phalanopsis bellina in June. I cut off the affected part and she's been fine since. However, today I looked and she's having the same issue again. But I don't get why because overall they seemed healthy. Although currently the bellina specifically looks a little rough. I have a timeline of my photos with labels. Please help me figure this out. It'll break my heart if I lose these 2. They're my favorite. the white stuff is diatomaceous earth for pests and use mosquito dunks for some gnats. And the issue is always at the tip of the orchid leaf.