Not the first time I have read of a problem with a (or any) Sekonic meter. Most problems relate to sudden death of the lithium batteries specified for use in Sekonic meters. Easily fixed, unless something pops up after that...
My first Sekonic L-758D was replaced after it went troppo right on cue of commencing a shoot. Now, the L-758D is a much higher spec'd device than the L-508, but the derangement of the meter has been explained to one (or more) causes as it has been seen in several manifestations locally. You are speaking of an oddity occurring with ISO selection, doubtless one of a few known manifestations of derangement. Possible causes: the meter has received a knock or a drop, even if it is in the pack. Not totally implausible also, the meter has been turned on (e.g. in the pack) with several buttons pressed concurrently from something pressing against it as a whole. Another is a sudden change of atmospheric conditions, particularly very dry to very humid, and/or very cold to very warm (resulting in condensation build-up inside the meter). For each and all of these conditions, changing the battery has no known corrective effect. On my meter the internal (spot meter) display became garbled, but I could incident-read. Others have experienced a failure of incident/spot meter selection or blanking of the ISO2 field.
If the fault is not recoverable I see the only option being a replacement meter. I did that, with an associated $800 hit to the cash balance!