(This is here in case it is the way Metamorph and Advanced Rocketry Interact with eachother, this is NOT genuinely the cause, it is to be believed AR's way of handling slow fall on low gravity planets is causing an inverse effect)
See issue Advanced-Rocketry/AdvancedRocketry#1291

Metamorph Version 1.1.5-1.12
Advanced Rocketry Version 1.4.0-80
Vulpes Library Versión 0.2.8-31

When In chicken morph while on space station the slow fall affect for the chicken gets inverted, causing the chicken morphed player to slowly fly upwards, this has not been tested on low gravity planets due to under development of the modpack.

(Find us over at github.com/grimco under CoopLife2)

Sorry for a long delay, but... what do you want me to do with it? Fix it? It's an incompatibility between mods. The only way to fix it, IMO, is disabling chicken's slow falling ability in morphs.json.

Honestly the problem is due to Minecraft's gravity code. There is no gravity parameter, so the only way to change an entity's gravity is to change its y-velocity. The proper way to fix this would be to add entity gravity to Forge, and have both Metamorph and AdvancedRocketry use that.

The dirty/unilateral but convenient fix would be to check if some other mod changed the y velocity in an unexpected way.

A less dirty way that doesn't involve Forge would be for Metamorph to add a getGravity() function to its API that AdvancedRocketry could check.

I forgot that this was open
WE could very well do that but our modpack depends on the mobs having such abilities (mainly chicken abilities). We would rather suffer than disable that.
This also affects minecraft chickens but who expects to take animals beyond the overworld (Oh wait I know a few....)
If there is nothing you can do about this please be my guest in closing this issue, we will just find a way around it.

If there is nothing you can do about this please be my guest in closing this issue, we will just find a way around it.

As far as I can see, this is the offender. IMO, the main problem is that chicken's ability is get applied before gravity gets applied in AR.

I think I can fix that by adding a check for negative motion over here.

I thank you @mchorse for helping to fix this issue. I honestly thought this was an AR issue and just linked it here to keep things 'organized'.
We have not heard any word from the AR dev in almost a month about any of our issues.
Heres the kicker:
We plan to remove AR (due to everything about AR) and move on to GalactiCraft, its more stable and has more capabilities.
This is in fact not an issue with GC, so it is up to you if you want to continue to fix this as you have said, or close this and forget this ever happened since it is such a specific issue.
(And thank you for putting up with me)

I will fix this issue, since it's seems to be very easy to fix, and IMO this condition is needed. I'll close it once I'll add this condition (anytime before releasing 1.1.6).