Unfortunately I have not yet been able to recreate this with a several of my X15. Our team is looking into these issues and should come out with a update soon. If you are still having issues please contact our support team for a warranty replacement.  

EVGA_James
Unfortunately I have not yet been able to recreate this with a several of my X15. Our team is looking into these issues and should come out with a update soon. If you are still having issues please contact our support team for a warranty replacement.  

Hey James, thanks for your reply. If it can help your team to replicate the issues, here's my specs:
 

 
Re-BAR is enabled in BIOS (I add this as I remember that USB had issues with PCIe 4 and AMD's chipset).
 
Since I downgraded the firmware to 1.28 I have to say that I haven't encountered any disconnections. I can provide you more information about the mouse if needed (S/N, date purchased if the product has been revised).
 
Hope this helps.

EVGA_James
Unfortunately I have not yet been able to recreate this with a several of my X15. Our team is looking into these issues and should come out with a update soon. If you are still having issues please contact our support team for a warranty replacement.  

sesquivel
Experiencing same issue. Mouse was under warranty so I was able to RMA it. Received a replacement (a refurbished mouse, instead of a new one ), same issue still happening. Mouse disconnects at random times. I don't have this issue with any other devices connected to my pc, so I don't think the issue is with my computer/usb ports. 

GREAT! Lets pay postage to send it in for RMA only to be sent another faulty mouse,
not only that but a refurbished one.
Been there, done that, lost money .... no thanks.

elvinmagical
I was able to downgrade my mouse firmware with the following steps:

1. Download EVGA Unleash RGB software version 1.0.13.0:
2. Extract the EXE file to a new folder using 7-Zip
3. Open Library\X15RGBLibrary.dll with a binary-safe text editor (Notepad++)
4. Search for the string "X15_v1.28.bin", which is the last good firmware file as of now (the file itself can be found in Firmware folder)
5. Look for the number after file name ("284"), it's the firmware's internal version and compared with the one from connected mouse
6. Modify it to become 291 (the latest/buggy firmware v1.34 has internal version of 290, according to Library\X15RGBLibrary.dll found in driver version 1.0.15.0)
7. Quit any running EVGA driver, run extracted UnleashRGB.exe, it will attempt a firmware "update" (downgrade to v1.28) because mouse-reported firmware version is 290, lower than 291
8. After finishing the update, the driver will attempt to update again (this time it's 284 < 291), cancel it
9. Voila! Your X15 mouse firmware is now downgraded to v1.28.

The 1.0.13.0 driver can then be installed normally. Don't forget to disable auto-update this time.
(EVGA, please don't patch it in a future release. We need this trick to prevent other undesired firmware updates done accidentally.)

Let me give this a try and see how it goes.
I was able to roll back to v1.28. Took me a few tries but I got it. Now time to test.

So far, no issues after role back of firmware driver(Knock on wood) to v1.28. Prior before the firmware role back this morning I tested the X15 with the current v1.34. The X15 mouse had dc'ed on me several times. About 15 times.  Since firmware role back to v1.28. The X15 mouse has been solid. 
 
Note: They attached image. Is what EVGA RGB looks like after the application crashes.

After downgrading the firmware to 1.28 the problem is still there
 

I too am having random disconnects with my X15 mouse.  I've tried multiple different USB ports, and also a separate USB hub to no avail.  The firmware is 1.35.
I have an ASUS Z690E Gaming Wifi mother board and have updated all the drivers as well as the BIOS in order to fix it, but nada....
 
I've tried looking in the event logs, but see nothing in there whenever the mouse disconnects.  I have just received a second X15 mouse that I can test to see if there's something wrong with my original mouse or not.

I just looked up the UnleashRGB 1.0.16 and the firmware is indeed updated from 1.34 to 1.35 for the firmware.  Is anyone else able to confirm issues as Aquatorch has mentioned (as well as iDrizzt in a separate topic)?

Well we got version .17 which seems to have the fix everyone is having issues on.  RETEST TIME!

New driver to fix disconnect EVGA UNLEASH RGB v1.0.17.0 (X20/X17/15/12 drivers)