SRAM RIVAL 1×11 and Garbaruk cage 11-50t indexing

Hi all gravelers!
I have a question about SEAM RIVAL, Garburuk cage extender and indexing.
I used to run the standard long case SRAM Rival 1×11 with a 40t chainring and a 10-42t cassette. But some of the local event around Squamish or Cowishan in BC use mountain bike and forest road with some serious inclines. I found this hack online to change the rear derailleur cage for the Garburuk to allow up to 50t cassette.
I paired this with a 11-50t Sunrace cassette and a new SRAM PC1130 120links.
After hours of adjustment of the B screw- changing the spacers at the chainring, I finally got the system to relax both end of the cassette without any issues and shifting was pretty good.
All this was done back in April before the Cowishan Crusher on Vancouver Island. But once in the race my shifting keeps jumping around. The chain will not stay on the large cog unless I keep pushing on the shifter, going down will jump 1 or 2 cogs so to go down 1 cog I will shift (drop 2) then shift back up to get to the cogs aligned with my desired cadence..
I did adjusted the shifting again after the race and can now stay on the large cog but the double drop when shifting from large to smaller cogs is not fixed yet.
To help describe the problem with other words, it is almost like the cable get stuck in the shifter then at some point after 1second – something move in the shifter then release the cable then the chain drop down. The shimmying doesn’t seem instant anymore.
I tried everything at the derailleur – change the derailleur hanger, got it check for alignment, played with the B, L and H screws…
I’m wondering if the issue doesn’t come from the shifter but I’m afraid to dig and open it up as those are hydraulic and I don’t want to mess it up.
Anyone experienced this issue? Any insight? Any lead on how to fix this?
I haven’t tried to revers to the regular SRAM 10-42t cassette and regular cage to see if that makes any difference. May do that over the week though
Thanks!!