Samsung S22 Ultra – playing on Realms server.
Brushing Suspicious Sand or Gravel does nothing.
Target circle does not fill while brushing.
Tried brushing from above, below and all available sides.
I've spent welll over a minute brushing suspicious blocks in multiple locations – not one has dropped an item/broken/fallen/whatever else. Block remains suspicious.
Shovel (or other tools/placing lamps etc) will make blocks fall.
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"Suspicious" sand or gravel does not refer to sand or gravel that has been generated in a strange way.
There are two specific block types named Suspicious Sand and Suspicious Gravel: https://minecraft.wiki/w/Suspicious_Block