Based on 1.13.1 decompiled using MCPConfig.
The method PlayerInteractionManager.tick() uses in the else if-block the position field delayedDestroyPos for determining the block hardness. This field is only set when finishing block breaking and therefore has at this code location an incorrect value. Instead the field destroyPos should be used.
This has currently no implications since there is no block which specifies its hardness based on its position.