G/L account substitution with invoice reverse

Hello Gurus,

We have posted an invoice which is replacing G/L account value with the help of substitution ( substitution name = 'FPD_FNL') in OBBHde in which we have used the user exit 'U408' with some fixed value say 'X' but when we are doing the reverse invoice in that case its not showing sustituted G/L account value.

I read on SCN that in case of reverse invoice ( Tcode is MR8M) possible.

If yes I just want to know the reason why substituion is not possible with reverse invoice.

Our user exit is triggering but its deleting the line item with substituted G/L account value for reverse invoice.

BR

Sumeet