The diags kernels have "diags" in the filename, and the main kernels do NOT have diags in the filename (just like fastboot help shows).
You need a compatible version of the diags kernel for your diags partition. Flashing a main kernel to onto the diags kernel will not work.
And you need an ENABLE_DIAGS file. Do you have one?
Did you try booting to diags with MfgTool?
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