There are hairs that we have treated multiple times over 50 hours. I know this because there are clusterings of thick coarse dark hairs in amongst a lot of fine light hair which I have been tweezing for over a decade, and some grow a certain way (curly, weird angle etc) that I remember because they were such a pain to tweeze.
These hairs sound like they are severely distorted hair follicles from repeated tweezing.
Thermolysis is almost, but not quite, useless against such hairs because you cannot tell where the follicle parts you need to destroy are, and may not be able to acess them even if you do. The solution is sometimes to use blend on these distorted follicles because the liquid lye accesses all parts of the follicle , whereas the heat from thermolysis we cant quite get there.
It's likely however your electrologist is proceeding in thermolysis to get rid of the bulk of hair that is NOT severely distorted, and will clean up these hairs at the end with blend, which might be a little less efficient for the large volume of hair. If distorted hairs are returning, its because they arent dead. They arent dead because they are severely distorted from plucking.Again this is not an unusual scenario.They can be killed.