The bug
Carvers can't carve through suspicious gravel, but they can carve through suspicious sand. This is because suspicious gravel is not in the #overworld_carver_replaceables
tag.
Linked issues
relates to
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Can you provide some examples where this was real impact? Thanks.

In my testings, I found a trail ruins structure generating on a cave. It seems that it generates after the carvers, so the carvers don't cut through any trail ruins blocks anyway (even suspicious sand, which is part of the #overworld_carver_replaceables
tag). So at least for trail ruins, it doesn't seem to have any gameplay impact.
I will test later if this is an issue for ocean ruins.
Confirmed.