I might be misunderstanding your question, but it seems like you have the format set to show 2 decimals. Then when the number is pasted into the cell then it trims it to 11.72% which is showing the 2 decimals that you have in the format. If you don't want it to round to 2 decimals, then set it to something besides 2 decimals. Anaplan will keep the full value that is entered, but in your format, this is how you have it defined. So your calculations based on that number would still be correct.
Might be i am wrong in understanding the issue, but what i understood is that the user copy the value from the calculated field in Anaplan and paste in another line item which is manual input and edit it to add a negative sign.
So probable user click F2 or Double Click to edit than there is a issue with Anaplan that it truncates two decimal points by its own.
For Eg :- Value copied from Calculated Cell is 12.34% and when you will copy paste this in Input Cell directly it will go fine as 12.34%.
But moment you press F2 or Double click and add "-" negative sign to it and press enter it will take as -1234.00%.
If this was the issue than the only option is to train the user to Manage decimal when editing percentage values or you create a line item and add "-"Calculates Field and ask the user to copy paste directly from this line item.