Actually I could experience as I had my case replaced for a slim case it has to do with the case design.
The marked area would have to be designed more even. It looks and is made to support (slim) plugs, but it has shown that it now even seem to be slightly more raised compared to the old case. This 1mm can make the difference if any plug connects reliable or not.
With my old cover any type of 3,5mm stereo plug fitted properly.
As I had my new slim case on I was wondering why all of a sudden I would only hear sound in mono not as it used to be in stereo. I investigated and found this cause. Without any case sound all is fine again.
I would not state that the plug sits too loose, I rather tend to say it sits quite fast.
So the 3d case design data is available, but up to now it does not look like anyone has changed them but rather tried to print a fitting new case.
Even I wonder if FP has taken this point into account as it just freshly got new transparent covers made. Is there anyone here who owns such a cover and can contribute his experience on this point?
Although it is the oldest and should be the easiest way of connecting any audio equipment, here it does not always work as expected.
It is not such a big problem for me as I use BT earbuds. But yes, this is not so power conserving.
Anyway, for home use I would still prefer to use a BT audio converter. I cannot imagine anyone wanting to hold his plugged handset while moving around his home. For this I found audio adaptors being compared here. I would prefer to go for the Logitech as a very good compromise, although it does not support the aptx standard. I believe audio being compressed in any way of .mp3/.ogg could not fulfill the high level anyway. Audio in form of Flac probably could do.