We also did online and not on the app - very easy. Although had to do it more than expected. Once for arrival from LHR as planned, then after going airside at SIN a couple of days later our flight to TPE was cancelled 1-2hrs post scheduled departure time and so had to hurriedly complete again to re-enter Singapore, before then leaving again 24hrs later.

We got taken aside by border control on the re-entry who unsurprisingly wanted to know why after spending 2 days in Singapore we then exited and tried to re-enter within the space of a few hours but all fine when explained, and no issue on the re-entry from TPE a week later (when of course we did the cards again).