Trouble whith F4311.NXTR update by P43214, when reversing 2 partial receipts of the same line

Hi !   I'm having an issue in the following scenario ( Enterprise One 9.1 plattform ) :

a- 1st partial receipt of a PO line ( line type B, non stock ) :  inserts a line in F43121 ( NLIN=1 ), with NXTR=380, LTTR=280.  Updates F4311 to NXTR=400, LTTR=380 ( previously was in NXTR=380, LTTR=280 )

b- 2nd partial receipt of the same PO line  :  inserts another line in F43121 ( NLIN=2 ), with NXTR=400, LTTR=380.  Updates F4311 to NXTR=900, LTTR=400

Everything fine so far, here´s the issue

c- Both lines are selected in P43214 ( checkbox ), and reversed at the same time.  Everything goes fine with the reversal, except for the update of F4311.PDNXTR.