I went to KL 2 weeks ago, so just a bit later than you. I found the MDAC system straightforward as far as filling in the details are concerned, and indeed got a reply by email, quite rapidly (cannot remember, maybe just a few hours, not more), with a pdf file attached. However, the pdf did not contain a QR code. I could get confirmation on the MDAV website that my application was indeed received, however, when going to get the QR code, the system replied with the message that "no record was found". I tried multiple times. I got a bit nervous, having to fly to KL without a QR code. There was absolutely no explanation whatsoever on the website why I would not get a QR code. So, rather poor show I think!
Arriving in KL, I could go through the automatic gates (EU passport) no pbm. Maybe that's where the MDAC system recognises me as having registered online? No idea!
Leaving KL, a bit of a nagging doubt again - what if immigration asks to see that damn QR code - which I never got? But I went through no question asked.
Conclusion, it's a good system. Makes entry-exit easy and quick. Shame it was a bit obscure wrt getting, or not, that (in)famous QR code.
Having had a few minor but annoying issues when registering for entering Thailand during COVID times, I rather expect the Thailand equivalent MDAC system to be even more obscure.
Ah, that's the word, is it not, about Thailand : obscure....