From what I know (or hopefully correctly perceive), that is totally wrong, as it is a conclusion based on wrong assumptions.
Fairphone is a small company selling most phones through their own website.
Just take a look at the number of phones sold.
Those phones are not produced like those by the global players in a constant stream on a production line (allowing for changes every day). Since those production lines do in one day deliver what Fairphone sells in a year, the phones are produced in “batches”.
This means:
While users started to complain, that the letters are falling off, Fairphone possibly has produced 2 or 3 batches and was working on developing the FP3+.
The FP3’s that are being sold, were most likely produced, before Fairphone even knew that the letters would come off.
As a kind of proof, take a look at the changes they have made to the FP3+. There are no more letters glued to the back. Seems, that the glue-solution could not be made to last and that they were working on this problem but could not solve it, before the last batch of FP3 was produced.