Nexus 6 - black screen on unlock
I am not certain if there is the Nexus 6 is really using magnetic sensor to determine if it is in a holster but your mention of noticing this problem after prolonged use with a magnetic clasp makes it likely.

If this is your problem then a refrigerator magnet is way too weak. Ideally, you want an AC current generated magnetic field, similar to the old days when one needs to deGauss a CRT. You can try using a magnetic setup that is used to erase computer/audo tapes.