The Canta range is one of the lower ranges of recorder and so the attention to detail that goes into making it, would not be the same as a hand-made recorder like the Mollenhauer “Modern”. So in answer to your question, it is basically poor workmanship in a factory when they knock ’em out cheaply.
If Ken is having the same issue with a Denner then I am surprised, since the ones I have tried have been really good on the F# (or B flat on an alto).
It is possible that the reedy not can be fixed by adjusting the size of hole 4 (counting the thumb hole as 0) but that would be have to be done by a professional making sure that the rest of the tuning is not affected.
As to whether this is financially viable for a cheap Canta is debatable.
The best tenor recorders I have found that have strong low notes are made by Fehr. In general their design starts with nice wide holes in the foot joint and they are then tuned from there. If you ever get the chance to try a Fehr before buying, go for it.
When I have drilled holes and tuned a recorder from scratch, the holes have to be gradually widened bit by bit until you get the correct pitch and sound.
This reedy sound will be on many notes during this process.
Hope that is remotely helpful.