I call it the Crown Heights "love affair" (for lack of better words) as opposed to the Chabad "love affair" because it is a hakpada that is primarily found amongst Chabad sofrim from (or trained in) Crown Heights, as opposed to Chabad sofrim from Erertz Yisroel and other places.
It is fair to say that different groups have different things they are machmir about (ie the Litvish are into klaf avodas yad, Satmar are into Bli Bechiros etc). And the fact that in recent years Chabad sofrim from Crown Heights are way more makpid when it comes to doing the tagei lamed correctly than anyone else (right higher than left - without exception - even when other hiddurim and lechatchillah aspects are blatently overlooked) IS A GOOD THING.
Here's why I say that they are more makpid than anyone else:
True, all good sofrim are makpid to write the tagei lamed properly - with the right tag taller than the left tag. After all, it is brought down in the Mishnah Brurah as a lechatchillah leksivah - just like the oketz on the middle head of the shin or the oketz on the back bottom of the aleph.
In Shulchan Aruch Harav it is not brought down as lechatchillah leksivah but as "sod'. This would be in the same vein as other kabballistic diyukim and hiddurim and most similar to the Shulchan Aruch Hrav's mention of the right side of the chottoteres of a ches being thicker than the left side. (The reason I say most similar is because they both come from the same kaballistic concept of chesed overriding gevurah).
At any rate all good sofrim will do it, across the board. (On a personal level I do not sell a mezuzah or parshiyos b'geder mehudar unless all tagei lamed are correct, and if they aren't, I fix them prior to sale.)
However what makes this breed of sofrim unique, is that when used mezuzos are brought for routine checking from people's homes, they will work on the tagei lamed. 99% of other maghihim simply will not do this. This is true to the extent that rabanim I have personally asked, in my own shimush, have said that a magiha checking used mezuzos is not obligated to do such tikunnim. (For this reason there were periods where I was not as makpid about it as I am today.)
And they will do it even on cheaper mezuzos which may not have all of the lechaltchillah leksivah requirements - and certainly not all the kaballah requirements.
Yet what puzzles me is that many of these magihim, who should be commended for their detail of the tagei lamed, will often ignore things that are of greater halachic significance! In my opinion, if you are going to fix to such perfection - fix everything!- and certainly things that are more chomur from a halachic point of view. It seems odd to fix all the tagei lamed but leave blatant cases of, for example, sofek kav moshuch or lamedim that have short necks - both of which are examples safek psul - which are of far greater concern than missing lechatchillah leksivah details.
And finally, for those who argue that because it's "sod" it has greater spiritual powers than halachic stringencies - why then are these magihim focussing only on the lamed and not the ches? They should place equal emphasis on the ches - which is also "sod' and carries the same spiritual rationale of chesed over gevurah. It is, after all, also brought down by the Alter Rebbe. Yet they do not give it the same emphasis. I see for myself many of these mezuzos where the lamedem has been oviously fixed (with bolet "I was here" tikunnim , the ches has not been touched.
Furthermore, one would think that from a spiritual point of view it is far better that the person has kosher lechatchillah mezuzos, than kosher bedieved mezuzos with fixed tagei lamed. Fix the bedieved's first!
Don't get me wrong - I think fixing the tagei lamed is important, and I do it all the time. However its important that everything is kept in correct context.
PS - ADDITIONAL THOUGHT: (someone sent me an email with the following argument). While halachically it is permitted to use a rapidograph for minor tikunnim such as oktzim and other lechatchillah leksivah touch-ups by a magiha, one could argue that if one is repairing tagei lamed because it's "nachon al pi hasod" it seems like a bit of an oxymoron to repair on that basis using metal, which is in itself is considered dubious al pi hasod. I should think that there is merit to this argument.