The Wikipedia page on Mostek, one of the original fabricators of Z80 chips, includes the following claim:
When Vin Prothro, President, and L. J. Sevin, Chairman of the Board, discovered that Zilog had modified the recipe for Z80 chips to keep the yields low, thereby buying Zilog time to build their own fabs, Mostek sought a new microprocessor partner.
The article is poorly referenced, and I haven't been able to find any corroboration of that claim.
The Computer History Museum's Zilog oral history includes a little about the Mostek relationship and the genesis of Zilog's own fab, and briefly mentions a problem with the first wafer - but (perhaps unsurprisingly) this particular issue doesn't come up.
So, (1) is there any corroboration of this claim? And (2) how might Zilog have 'modified the recipe'?