What kind of probes are we talking about? Most probes we use in the hobby (and elsewhere for that matter) work on the dissimilar metal principle where two different metals bonded together and placed in a liquid will produce a small DC voltage. This voltage is then amplified & displayed numerically on a digital meter, or in the old days a mechanical meter movement.
I'd say an inadequately shielded AC device (powerhead or heater) in the proximity of one of these probes could cause some issues since the changing magnetic field of an AC voltage might induce noise into the small signal produced by the probe. A stationary magnet might also affect it, but as long as it's not moving, there's no change in the magnetic field around it. This & the fact that most of the magnetism will be concentrated towards the magnet or piece of metal on the bracket, should make interference less likely. Blah, blah, blah, just a few little ideas & theories I'm throwing out there from my past electrical & basic physics learning days.
I guess the best way to find out in your particular situation is to place the probe without magnet holder in whatever you're wanting to measure and check the reading on the display receiving the signal, then repeat the procedure with the magnet holder. If there's a significant difference, you'll have your answer. If not, well you still have an answer. Do the same with any other probe you're concerned about.
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