recorded land in one's name is transferred to another without informing the legal owners.

In the year 2003 i.e.19 years ago my father bought a land which was owned by 5 people but in the registration copy signature of one person is missing . That land was recorded in 2015 i.e.after 12 years but it is in our possession.We have no information about the deal.Since my father's death from 2021 one of the heirs of that absentee owner is disturbing us for that land and as the signature of that person in missing he is putting pressure on us for transferring it to him. We have lodged complain to BLRO and Police Station but nobody is responding. We have all the legal copies of that land and the initial dealing statement for purchasing the land where a small amount was exchanged between all the 5 members is also with us. RI investigated the land in 2015 and thereafter it was recorded in my father's name and we have the copy also. A few days back a portion of that land is recorded in the name of that absentee owner and without informing us in our absence as we were out of station at that time that land was investigated that we came to know shortly. There was no issue with that land in last 17 years but after my father's death all these happened. As we are unaware of the deal and what happened or not at that time that person is misinterpreting everything. What should we do in this situation ? Please anybody show us the right path.Thank you.