ALL scars stretch Joe. all of them! some worse then others obviously.
rare? well, ppl come up on this forum monthly complaining about their FUT scar and ask for advice on how to repair it and which FUE doctor to chose to fix it so its not quite as are as you say.
where the donor tissue is extracted also plays a role on the stretching. some lucky strip hair transplant patients will have only a slight paper thin scar while others will develop thick, protruding scars 1/2 an inch in width or more.
its almost impossible to determine how individual patients will heal, and whether their scars will stretch over time but overwhelming evidence shows that MANY ppl who hav FUT will end of with ugly tell-tale scars and will never be able to cut their hair short.
friend of mine had FUT from a pretty good doctor (tried steering him clear of FUT but he knew everything so I said have at it) and after the first year the scar was fine. the following year the scar stretched pretty bad and the year after that it stretched so bad he had to have a revision. but, he did have a family history of healing poorly scars so not exactly apples to apples. I tried warning him....
that may be rare and probably after 4-5 months if the scar has not stretched considerable it probably won't stretch much further but it can and does happen.
and there have been plenty of ppl post here years later looking for help on how to repair their FUT scar. over time scars will flatten but I have a scar on my chest from being stabbed and it looked great for the first couple years and years later it stretched at least another 1/2 inch. 1/2 inch may not sound like much but on the back of ur head its a whole bunch!
granted, the scalp heals differently then the body but nonetheless it continued to stretch over the years.
my buddy ended up going with a decent US FUE doctor to fill in the scar after it healed. it looks OK now. nothn special tho. a trained eye will spot it but most ppl wud probably never even notice.
he says he shud hav listened to me. and the doctor told him he wud suture him up really tight and to have no worries about the scar. well, so much for that.....
the only sure way to avoid having a linear strip scar that is noticable is to elect not to have strip surgery.