Disclaimer: Backup your saves before running any new version of Factorio, any changes in mods, or changes mentioned in this post. Any loss is on you for following any of my advice.
Rampant appears to be the last of my heavily modded games broken by the recent changes (v0.18.36) to the slowdown capsule graphics. I extracted the slowdown capsule assignment data from the base game (data\base\prototypes\entity\entities.lua lines 5047 through 5066 inclusive) and copy-pasted it into Rampant (Rampant_0.18.16\prototypes\SwarmUtils.lua lines 1029 through 1049). Make sure you preserve the code blocks (open and close curly braces {}). This change allowed me to load a save game with "World: Enable new enemies" still enabled, and I was able to test using the slowdown capsule on bitters that still spawned.
No further support will be offered for this change. I am way to drunk to follow this post and I want to play Factorio damit!
Thank you Veden for all your hard work! Thank you slippycheeze for your efforts in finding the problem.