Sandeep, Confirmed case of 3rd party processing with TAB item category. Let me try my best to see if i can help further.
Simplest thing is that you do not have to maintain any Strategy group in material master.
Have you set up any thing there for the str.grp in MRP view? If yes, delete the same in Material master and repeat the process.
Availability check as pointed out is not important rather it is the transfer of requirements which is relevant now. Is the transfer of reqmts indicator checked ON for your sch.line category ZG?
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Clean solution would be to ensure that the TOR does not happen past SD @ SL level. Check with your SD consultant further. In short this scenario is not relevant to MRP and settings need to be controlled from SD end.