Do the magnets in iPhone belt holsters cause phone problems?

I never had a problem with it.

I've heard some people say that they would stay away from cases with magnets in them, and then others say that the magnets should not hurt the iPhone. Personally I keep magnets away from mine

From a theoretical perspective the only things a magnet could affect are the speaker, mike and (with a bit of a stretch) the ferrite core of the antenna if it has one. A magnet should not bother the microprocessor or memory. Most well-made cases with magnetic clasps have a shield between the magnet and the phone.

After going through 3 iPhone's in the last month I would tell you the answer is yes. And if you don't believe me visit an Apple Store Genius Bar (like I have had to do), and have him walk through his iPhone troubleshooting manual for things like "corrupted SIM cards" and you will find written documentation from Apple to avoid magnetic clasps.

This reply from Beyaza Cases:
Thank you for your inquiry.
Magnets used on the cases are not harmful and does not effect your device.
Also, we have never received any complains from our customers who bought the case you own.
Please do not hesitate to contact me anytime.
I've seen similar replies posted here from Sena. Apparently only strong rare earth or neosymium magnets cause issues.
Anyone here using a Beyaza pouch and magnetic flap with an iPhone?