Internal SSD not recognized but working fine

Hey Swarm!

I would be happy about some tips for this problem i have with my macbook. I just exchanged my DC-In-Board (Magsafe Board) as my mac was not starting up anymore (just black screen, no matter what). The board had two burned pins and was getting pretty hot, but was working like this for some months before it finally stopped. After exchanging the dc-in-board the mac was powering up again, but the next problem appeared: a blinking questionmark, indicating, that there is no drive available to read the OS. Before the exchange of the dc-in board the drive was just working. I already tested a lot of things, but the results dont make any sense to me: it seems like the hardware is completely working and i can even access the ssd and boot my system from it, if i connect it externaly.

Tests:

• PRAM reset
• Boot in recovery mode but no drive is shown neither in disk utility nor in the boot-menu
• tested the ssd (+ssd to nvme adapter) on another mac book of the same type (my brother owns the same macbook) = also blinking questionmark
• tested ssd with a new ssd to nvme adapter = also blinking questionmark
• connected ssd via an external enclosure on a third mac - ssd was working and showed all of my data
• connected ssd via an external enclosure on my mac - booted up normally without even asking if it should boot from this device
• ran first aid in disk utility twice (once on my mac once on another mac), it did some fixings in the process, but still didnt work afterwards, drive seems to be ok

Specs:

• Macbook Pro 15 inch Retina Mid 2015
• SSD Drive: HX2TSSDM2PCIE by XUM, 2tb, PCIE, M.2 (comment: when i bought the mac, the screw fixing the drive was missing, after two years the drive came loose, so i already had the blinking questionmark some months ago, which was easy to fix, by just pushing the ssd back in and adding a screw)

So now im a bit confused. I can only think of, that i have three ssd to nvme adapters that are not working or any strange software issue but honestly - no clue. Anyone any ideas what could be the problem? I would apprecieate any help or tips. Thanks in advance <3 <3 <3

Greetings

Alex