I have a gravel bike with a SRAM Apex 1x RD and a 11-42 SRAM Apex cassette that has never worked that well, but lately it's becoming exasperating: it refuses to upshift from the 4th to the 5th sprocket. If I adjust it so that it barely upshifts, then, when on the 2nd sprocket it will randomly fall down to the 3rd, and when on the smaller cogs, it will not downshift. Also, when on the smaller cogs, if I downshift more than one cog, it simply refuses to downshift all of them (if I click twice, it downshifts once. If I click 3, it downshifts 2). It's specially frustrating because the shifters being doubletap, it's impossible to "nudge it" to force it to shift, as that clicks for an upshift inevitably. Sometimes it shifts better than others for no apparent reason, sometimes it even (almost) works for a while.

Things that I have done:
- Checked the hanger alignment (that seemed like the most obvious thing) with a hanger alignment tool.
- Replaced all cables and outers TWICE. First time with Jagwire, second time with SRAM cables. I don't want to do it again since each time I have to replace 2 layers of bar tape and that's costly.
- Checked that the routing is good, with large radius curves, and that the cable moves smoothly (it does when pulling by hand).
- Checked that the shifter is releasing and pulling cable properly.
- Checked that the derailleur moves freely from side to side.
- Checked that the derailleur has no play anywhere.
- Checked for B screw proper adjustment (it improves if I move it closer than what SRAM recommends, but the downshift to the first cog is not ideal then).
- Replaced the chain. I'm using original SRAM 11sp chains.
- Replaced the BB cable guide just in case...

I'm about to trash the whole groupset, I have a Rival 1x11 in the road bike that is not as nice as the 12sp on my mountain bike, but at least it works. I've even had bikes with Shimano SIS and Microshift, and I never found I bike that shifted as bad as this one.

Any idea?