A small subset of the reproducer shows that invalid code is generated:

program transfer_p
  implicit none
  integer, parameter :: n = 3
  class(*), allocatable :: o(:)
  integer               :: v(n)
  integer               :: i

  v = [(i, i=1,n)]
  allocate(o, source=v)
  print *, "size (o) =", size (o)
  print *, transfer(o, v)
end

This compiles and gives:

 size (o) =           3
           1           2           3           0           0           0

while compiling with -fdefault-integer-8 gives the correct answer:

 size (o) =                    3
                    1                    2                    3

A look at the dump appears to give some hint where to look further.

Another datapoint: inserting

  select type (o)
  type is (integer)
     print *, transfer(o, v)
  end select

prints the right thing:

           1           2           3

Also note the possibly related PR84006, where one gets an ICE on

     print *, "storage_size (o) =", storage_size (o) ! ICE

unless one places this inside a select type.

Confirmed from GGC7 up to master.