NanoVNA uses si5351 above it's spec and most chips work fine above 200MHz and up to 300MHz, but some are a few MHz short of it.
This has been discussed here before and the key is the point where the vna transfers from fundamental mode to harmonic mixing mode.
Lowering the transition frequency from 300MHz to 296MHz for example fixed the 300MHz spikes for some.
Ismo V??n?nen OH2FTG