Arduino FastLed WS2821b led problem. Help finding the problem please.

First of all I want to say that i'm not an expert not even a novice in this topic. I made this project following a youtube video. I don't even know what was i doing, but it worked fine for some weeks.

I followed this (Youtube link) tutorial to make a monitor ambilight using an Arduino Nano (ATMEGA328P - CH340G) FastLed library and Prismatik to screen grab my pc screen.

My wiring looks like this except the dc plug im using a 5V 75W Meanwell "power box" (Meanwell LRS-75-5) to power the strip.

My problem is that if I turn the brightness to 100% the upper led strip turns yellow and the other led strips start to glitch. Colors showing wrong on the top strip too. So some led colors don't receive enough power to be turned on? Must be a problem with power?

The problem looks like this:

Furthermore if i lower the overall brightness to 20% the led strip starts to work normally. Every strip showing equally white and showing correct colors.

However the "power box" should be plenty powerful for the led strips. It's 126 leds long so 126 x 60mah is 7560mah which is 7.5A, this "power box" can supply 14A which is the double what I need... I feed the strips with power on both ends.

I'm using this led strip: WS2812B strip
The strips are joined by these solderless connectors: 90 degree Solderless L connector

I thought that the upper led strip must be damaged so I changed it to a new one but the problem remains the same. Maybe some power or voltage is lost in 90 degree connectors? To me it sounds silly that only the upper led strip don't receive enough power....

The next logical step would be to solder the led strips together?

Sorry for my post being somewhat hectic. I'm not native english so there's must be some silly sentences and grammar, but I hope you can understand my problem and help me.

Are you applying power to just one end of the strip? Try applying it to both ends, or the ends and middle if the strip is really long.

I'm applying power to both ends. The strip is not that long though. It's 126 leds so at 60leds/meter is just 2 meters long overall. 2 stips at the bottom are ~40cm - 40cm, two side strips are ~35cm - 35cm and the top one is ~ 80 cm.

I'd say insufficient power supply, too thin of a wire from the supply to the strip ends, or perhaps just low quality strip.
Does anything feel warm at higher brightnesses? That's probably the weak link in the chain.

Interesting... The bottom left and right L connectors get pretty warm but the led strips on those connector ends are working properly. The upper L connectors where are the "faulty" led strip are not warm at all.