HSN Code in STO process is updating the BSEG table, where as in Sales transactions it is not updating we have gone through all SAP GST Notes
Create a custom enhancement for the badi J1IENHAN_INDIA_GST_POSTING. SAP has given this badi, but did not give the enhancement implementation. You need to create an enhancement implementation for the same. Check the screen shot.
The India tab will be visible if below notes are applied fully and correctly with all the manual settings done as well.
note 2463381 - GST India: Corrections to External Service Procurement
Note 2444868 - GST India: External Service Management
Also please review note:2474348 - GST India - Control code field value is not auto populated from material master under INDIA tab in Purchase Order
•Make sure variable 'ex_gst_rele' is marked as 'X' in funtion module J_1IG_DATE_CHECK, so that control code logic gets trigerred.
•Within function module J_1IG_DATE_CHECK ensure any conditions based on variable sy-datum use the 'GE' or '<' operator rather than '=
For material POs where HSN code is already updated in material master this value will be default automatically and will be non editable.
Hope this helps