If rails, redstone repeaters or redstone comparators find themselves in a position they cannot naturally be placed in, and receive a block update, they will break. According to the comments in MC-129127, this should not be the case, and the block should remain a block and not break.
Steps to Reproduce:
Set a comparator in the air:
/setblock ~ ~4 ~ minecraft:comparatorPlace any block next to the comparator, updating it
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Actually, since 1.13 the "support" block has to change for every block, in order for it to check if it's in a valid position; this is also the case for fence/pane/wall connections.
Avoma
Can confirm in 20w51a.
muzikbike
Placing the block above them causing them to break is a bug, but placing blocks next to them should still cause them to break, since it makes contact with the placed block on the sides and should thus update them.