yes, looks like a bug. I've modified the source a little to make it more clear where the bug is happening:

program pe

type, abstract :: t
integer :: i = 1
class(t), pointer :: next => null()
integer :: j = 2
end type

type, extends(t) :: u
integer :: k = 3
end type

type, extends(t) :: v
integer :: l = 4
end type

class(t), pointer :: ini => null()
class(t), pointer :: p => null()
class(t), pointer :: temp => null()

allocate(u :: ini)
allocate(v :: ini%next)
allocate(u :: ini%next%next)
allocate(v :: ini%next%next%next)

print *, '---'

p => ini

do while (associated(p))
print *, "current element", loc(p),"next associated?-->",associated(p)
print *, " traverse to next element "
p => p%next
print *, "next element", loc(p),"next associated?-->",associated(p)
print *, " "
enddo

print*, "exited loop - why?"
print *, '---'

p => ini

do while (associated(p))
print *, "current element", loc(p),"next associated?-->",associated(p)
print *, " traverse to next element "
temp => p%next
p => temp
print *, "next element", loc(p),"next associated?-->",associated(p)
print *, " "
enddo

print *, '---'

deallocate(ini%next%next%next)
deallocate(ini%next%next)
deallocate(ini%next)
deallocate(ini)

end program

I'll start a bug report

bug report ID is DPD200163749

btw - this is a very clean and nice example of using F03 to create linked lists! Great use of F03 features.

ron