When using foil tape you will have issues with trapping the heat if you cover the probe with the tape. The proper way to secure it is to tape the probe wire just before it meets the probe. Then if necessary you can use a thin piece to hold the tip of the probe down. You want to avoid ever securing the probe in a manner that covers the probes body.
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I forgot to ask.
How are you checking the hotspot surface temps in the rack?