cubicweb #3064653 regression introduced in some cases of inlined forms [validation pending]

eg in our internal activity tracker based on the gingouz cube, we can't create Order anymore, with the message "split_into-object: required field", while the value appears in the post.

priorityimportant
typebug
done in3.17.6
load0.200
load left0.000
closed by#312062f53981 [inlined form field] fix regression introduced in 3.16.4/570208f74a84. Closes #3064653
patch[inlined form field] fix regression introduced in 3.16.4/570208f74a84. Closes #3064653 [applied]