THE ANSWER
It is a documented effect called paradoxical hypertrichosis. In a minority of patients, laser hair removal triggers more hair growth rather than less, usually finer and darker, and most often on the face. It is real, it is in the medical literature, and it is not something you imagined or caused.
Why
The honest position is that the mechanism is not fully settled. The explanation most often proposed is that energy which is too low to destroy a follicle can instead stimulate it. Fine vellus follicles that were producing barely visible hair are pushed into producing coarse, pigmented hair instead.
That fits what is observed. It happens more at lower energy levels, more at the edges of treated areas where energy tails off, and more in people whose follicles are already responsive to a hormonal signal.
The risk is not spread evenly. Published work associates it with facial treatment, with darker skin tones, and with hormonal conditions including PCOS. Those three factors overlap heavily with the people most likely to seek facial hair removal in the first place, which is part of why this affects the group least able to shrug it off.
Reported rates vary a great deal between studies, from well under one percent in some to considerably higher in others. What that spread tells you is that the effect is uncommon but genuine, and that anyone quoting you a precise figure is being more confident than the evidence allows.
Why is there more hair after laser?
The working explanation is stimulation rather than destruction: energy that was too low to finish a follicle prompted it to produce a heavier hair instead. It is most often seen on the face, at treated edges, and in people with a hormonal driver.
Is paradoxical hypertrichosis permanent?
The new growth does not disappear on its own. Some reports suggest continued laser at adjusted settings can eventually reduce it, and others do not. What is clear is that the new hair is real hair from real follicles, and treating those follicles directly is the approach that does not depend on the pigment question at all.
Does this happen to everyone?
No. It affects a minority of people treated. Most laser patients see reduction, which is what the treatment is cleared for. The point is not that laser usually backfires, it is that when it does, people are frequently told it cannot happen.
What the evidence says
Paradoxical hypertrichosis after laser epilation is described in the peer-reviewed literature, including work indexed in PubMed examining the factors that induce it and studies looking at how often it occurs. Reported incidence differs substantially between studies. Higher risk is associated with facial treatment, darker skin tones and hormonal conditions such as PCOS. This page states that it is documented and does not put a single number on how common it is, because the published figures do not support one.
What to do next
Stop assuming more of the same treatment will fix it, and get the new growth looked at on its own terms. This is not ordinary leftover hair and it should not be treated as though a few more laser sessions will clear it.
If you have PCOS, see hair removal for PCOS, which covers the same ground for that group. If your skin is darker, see hair removal for darker skin tones. For assessment of what is actually there now, see electrolysis after laser hair removal.