well, from my sat & sun transactions, found that Adonis's posting date is 1 day later - in this case, swipe on sat with posting date on sunday ...
so, supposing if i swipe on sunday instead, perhaps the mentioned merchant will process it the next day, which is monday (1-july), hence may affect the cashback into the following month, july ... anyway, i'm not sure but just guessing based on my transaction record ...
i've mailed to Amex customer service & called Maybank - they assured me as long as transactions are made on weekend (sat or sun), you're entitled to 5% cashback, regardless of the posting date ...
so, i'm curious with the case study of Dell & Agoda above, since the transaction is done on weekend, by right should get 5% cashback regardless ... but then such online transaction may be different

You gave Agoda/Dell your AMEX CC details DOES NOT MEAN the transaction is done on weekend.
Someone at Agoda/Dell need to "process those details"... and they only work Mondays to Fridays