MultiVariantWalkerGroupedOnStart finds a variant that does not share a start with the
currentVariants, it processes that group of variants. However, it does so using the
readsContext of the found variant with a different start and not that of the group of processed variants. Furthermore, in
afterTraverse it processes any remaining variants with an empty
ReadsContext, which is just wrong.
This has not been caught before because no existing implementations use reads.