This is why people say that you pretty much have no choice but to max out your morale and economic passive bonuses when setting up your race.  I'm surprised to hear you have this issue even with the Korx though.

I would check the population of your planets.  How large are they?  Generally about 16-20 billion is as high as you want to go for a civilization capital, and 12-16 billion for others, because planets suffer huge morale penalties for 'overcrowding'.  For instance, placing the Thalan's Hyperion Matrix on a +300% farm tile ruins the planet (makes your population capacity 40 billion).  If your population is huge, consider destroying some farms.

If your population isn't a problem, then I don't know what to tell you.

How high are you trying to keep your approval?

Sorry, was going to check on this, but got sidetracked.

And I see you've found and solved your problem, which I didn't notice because I'd kept the page open.  You might be able to use some or all of my post regardless, so I've left it intact.

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Here's what I'm seeing, trying to keep things as close as possible to yours:

1. +10% morale with customization points
2. Xeno Entertainment (+15% morale) for multimedia centers
3. +50% planetary morale (initial colony + multimedia center + multimedia center & entertainment network to emulate festival of capitalism)
4. 14% taxes

14B: 99% (100 base - 40* population - 14 taxes + 29 buildings + 14 civilization ability + 10 high quality planet)
16B: 85% (100 base - 48* population - 14 taxes + 25 buildings + 12 civilization ability + 10 high quality planet)
18B: 70% (100 base - 57* population - 14 taxes + 21 buildings + 10 civilization ability + 10 high quality planet)
20B: 54% (100 base - 66* population - 14 taxes + 16 buildings + 8 civilization ability + 10 high quality planet)

For comparison, my homeworld (25% civ capital, 30% 2x multimedia center, 10% entertainment network):

16B: 93% (100 base - 48* population - 14 taxes + 33 buildings + 12 civilization ability + 10 high quality planet)
18B: 76% (100 base - 57* population - 14 taxes + 27 buildings + 10 civilization ability + 10 high quality planet)
20B: 59% (100 base - 66* population - 14 taxes + 21 buildings + 8 civilization ability + 10 high quality planet)

I've placed an asterisk next to the population figure for a very good reason, and one that you might have noticed if you look at the above numbers-the morale effect goes down from the same morale as population rises.  All positive morale sources are multiplied against your planetary population's malus before determining its approval, with the sole exception of the bonus for having a high quality planet, which is a straight approval modifier.

To my knowledge, the Korx don't actually get morale bonuses in their tree-and others are missing them as well in TA, actually-but perhaps more to the point everyone but the Terrans, minor races, and Arceans miss out on Ultra Spices and Virtual Reality Modules (which is an extra 27% there), leaving them with Harmony Crystals at 20% from Xeno Ethics and Frictionless Clothing at 10% from Xeno Cultural Trends (Historical Assimilation and Cultural Domination required), where applicable.

So it would seem the morale/population rule needs to be modified for TA: Don't go over one farm (14B).

For what it's worth, the above 14B planet and 16B homeworld are at 57% and 50% approval respectively at 49 percent taxes.