When breaking a slime block while it is being moved by a piston there is the possibility of the particles (visually) being lava particles.
I'm unsure about the timing, but as shown in the below video, it can happen and does require a few attempts.
This is similar to the other tickets relating to "wrong particles with block moving" however this block does not display the piston "head" particles.
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You are playing on what appears to be a broken resource pack. Please test this again on the default texture pack, and see if it occurs there.
Still occurs on default texture pack, as attached to the original issue.
Confirmed.