but having to spend half an hour to an hour of DPD processing
It took me less than 3 mins, would be quicker if you have a friend to hold the passport open for you while you photograph. You really shouldnt let the DPD stop you travelling.
I think its because the Plus versions (being a phablet) often get the tablet version of the app and the testing concentrates more on the phone version. For about a month the Qantas app stopped working on my tablet but then righted itself with the next update and my galaxy tab is older than the S21.
When I used to manage phone app development projects, registering an android app for the Google Play store, required you to specifically list all handset models and OS versions (this is why sometime overseas purchased variants dont work optimially with Aus app store). It could be that they didn't test the S21+ properly, or put in under the wrong app version. For iphones, apple themselves will sometimes dictate whether you can make a new app available for older models in their store (Apple have that whole forced obscelence thing going on).
We always priorirised the Samsung S series and the upper end iPhones. If time then theyd do the lower end Samsung (always A before J) and lower end apple (i.e the SEs) then some of the lesser androids. Whilst we tended to do phone specific versions of app for the flagship models (S series and Iphone), for other handsets they often would build an app that just rendered the mobile version of the website.
When Im due for a new phone I always buy the latest in the flagship series, as these tend to be supported longer than the cheáper variants. If you buy direct form Samsung or Apple you can get handsets which dont have all the Telco (Tesltra, Optus, Voda) bloatware on them, and depending on your employer can often get good discounts. Because of Telco bloatware, when OS updates come out you get them later on phones from Telco because they have to test their bloatware.
If you want an Android with no real bloatware (not telco or brand) then consider a Pixel - those phones now come direct from Google and only have native android.