I recently also stumbled over the spikes and scrambled mesh. I could track it down to using the undo function while in sculpt mode (and I think it was also relevant if it was a relative of absolute blend shape, but didn’t fully evaluate that). When undoing it still looks all good. You continue working, but then when leaving sculpt mode, it messes things up due to the undo step you did. However I think this was 2.83, not 2.9x, so this info might be outdated already 
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