[TYPO3] TCA-field 'group' for fileupload on flexform-sheets for different types only works with workaround, bug?

[Johannes Konert]

Dear list-members,
I am not experienced enough to judge whether it is a bug, a feature ;-) 
or my stupidity:
My extension makes use of a type-field to generate different flexforms 
for each one. I do produce them using dynaflex, but this should make no 
difference in this case. Now the bug:
If there are file-upload (or selection) fields, let's say on several 
sheets within a flexform for type 3, then the upload to upload-folder, 
thumbnails and all this only works, if the same field (same name) exists 
within the flexform for type 0 at the time of 'save'. If a so named 
field is not in the flexform for type 0 (default) then the 
TCA-Backend-processing just puts the absolute path (/var/www/typo3/..) 
into the field, does not copy it to upload-folder and thumbnail does not 
work.

So my workaround was to set the flexform for type 0 (default) always the 
same as the currently selected type (e.g 3), because the BE-user won't 
see the type 0 form anyway and TCA-processing is happily working when 
the save-button is pressed. But this behaviour is strange.

Someone experienced this before?
Did you got, what I mean or should I provide screenshots as an example?

Regards
Johannes
PS: Would typo3.dev be a better place to post this or ext-dev-related 
things in future?