Issue with x264 grain retention around edges (possibly bug?)

[TomArrow]

I hope this is the right forum, first timer here.

Trying to encode something with the following settings:

x264.2851kMod.10bit.x86.exe --preset=veryslow --tune=grain --b-adapt=2 --deblock=-4:-4 --rc-lookahead=60 --keyint=400 --min-keyint=40 --me=umh --merange=64 --psy-rd=1.5:0.15 --aq-mode=1 --qpstep=40 --partitions=all --no-dct-decimate --bframes=16 --no-dct-decimate --no-fast-pskip --deadzone-inter 0 --deadzone-intra 0 --aq-strength=1.0 --trellis=2 --qcomp=1 --subme=11 --ref=8 --crf 24.0 --output="Escape-Trailer-test.264" --profile high444  --input-csp i444 --output-csp i444 --input-res 1920x796 --input-depth=16 --frames 3601 --fps=24000/1001 -

There's no input file because I'm piping from ffmpeg.

Anyway, this is an issue I had with multiple encodes in the past, it's nothing 4:4:4 exclusive.

The issue is that while grain is usually retained very nicely, this simply doesn't seem to work when there are edges in the image. Worst example is a sharp text overlay. Around this text, everything is a blurry mess and whatever little grain structure is preserved there, is static and doesn't "move" like grain should.

Lower CRF values do help it, but dramatically increase bitrate, which I feel is unjustified considering that the rest of the image looks fine.

Now mind you, it's not so horrible as to be noticeable from a normal viewing distance on my LCD, but I'm an old pixel peeper and it annoys me, because it's just ugly. Like a part of the image around the text is static while it should be full of grain.

Any clues?