Factorio 0.18.6, Rampant 0.18.4, Rampant Arsenal 0.18.0, Honk 4.1.0.

When driving a tank with decent defences, the slowdown effects of spitters and worms stack, very quickly causing the tank to barely move at all or to stand still completely. If you just hold "w" and the attack continues, suddenly the tank starts teleporting many chunks away. The signs point to overflow/underflow of some slowdown calculation somewhere, causing absolutely massive negative slowdown aka teleportation.

In vanilla, slowdown effect do not appear to stack. Only the worst effect seems to apply, which is 30% for behemoth spitters and worms. Is the stacking of slowdown in rampant a bug or is it intended?

Savegame scenario: https://mel.vin/tmp/rampant_tank_teleport_save.zip

The tank is marked as invincible. Opening the map shows example teleportation path. To reproduce simply enter the tank, drive to the big nest north-west and hold "w". Eventually you will teleport. Factorio will space out a bit for mapgen and sound glitches shortly, no crash or similar.

If you stand still when under heavy attack on purpose for a few seconds before pressing "w", teleportation distance will be absolutely huge.

With some more playing I hope the stacking is a bug and not intentional. If even a few spitters shoot at you a few times the vehicle cannot move at all, even though even a basic shield setup prevents any damage. It's a bit tedious to wait before you can then move again.

Thanks for the report.
Not intentional. Will add it to the todo list.