I have recently had similar problems and issues with my tomato plants, - and I have the same climate as you have (Northern Thailand). Monsoon season was very hot this year due to lack of rainfalls and I suspect the high temperatures (too much sun exposure also) to be the major issue here. Especially too high night temperatures. My plants did actually recover during longer rain periods (lower temperatures), lately. I'd recommend a few hours (best is early morning) of direct sunlight only. Best would even be to limit main tomato culture during the coolest season (nov-feb) here, I guess. Or perhaps look out for more heat resistant cultivars. I have a tasty semi-wild variety (local) that copes pretty well with the heat.
Cheers,
Luches
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