I really don't know if there is a way of preventing it............I've seen this on a few of my own Tamiya kits.
It appears the "crack" is actually where the molten rubber/plactic of the tire flows through the mold and meets itself on the opposite side. The molten rubber doesn't completely attach to itself, resulting in the cracked tires.
I've done three different things (with success) -
1. I write to Tamiya, explaining the problem and including a damaged tire - they usually send a new set back at no charge.
2. Order and buy a set from the Australian distributor of Tamiya.
3. Make molds and cast replacements in resin.
All three have worked for me in the past - and some Tamiya cars and armored cars I've built look ok for years, then the tires crack, and some have never cracked. I can never tell which ones will be next.
Cheers
LeeTree