Still present in 4.2.0

pkg load quaternion
q = quaternion(1,1,1)
save("-text","q.txt","q")
save("-v7","q_v7.mat","q")
exit

now start octave and do

octave-gui:1> load q.txt
warning: no constructor for class quaternion
octave-gui:2> load q_v7.mat
warning: no constructor for class quaternion
warning: load: element has been converted to a structure
--verbatim

To reproduce:
 - create an object of a class (e.g. "quaternion")
 - save object to file
 - exit octave
 - start octave
 - load file

I created a custom object "transform" which has a data member "quaternion" (from the forge package) and ran into this behaviour. If I call the constructor before attempting to load the file it works as it should.

octave:1> default_save_options ('-text')
octave:2> load test.txt
error: Invalid call to quaternion.  Correct usage is:

 -- Function File: Q = quaternion (W)
 -- Function File: Q = quaternion (X, Y, Z)
 -- Function File: Q = quaternion (W, X, Y, Z)


warning: call to constructor for class quaternion failed

Similarly when I load a single "transform" saved in the matlab binary format:

octave:1> default_save_options
ans = -mat7-binary
octave:2> load test.mat
error: Invalid call to quaternion.  Correct usage is:

 -- Function File: Q = quaternion (W)
 -- Function File: Q = quaternion (X, Y, Z)
 -- Function File: Q = quaternion (W, X, Y, Z)


warning: call to constructor for class quaternion failed
warning: load: element has been converted to a structure
warning: call to constructor for class transform failed
warning: load: element has been converted to a structure