It doesn't look at all like a camera problem, and the fact that you used two cameras over 5 films does seem to make the only common factor to be a lab issue. I'd take it up with the lab right away.
(I've had occasional similar (and other) problems in the past with so-called "professional" labs in the U.K. The best ones seemed to be those used by manufacturers for their films sold process-paid, but almost none of those left now. I have some E4 slides taken by my late Father in the 1960's, with the mounts printed "Processed by Kodak", and these are still immaculate, with none of the fading which has happened with a lot of E4.)