Not sure if intended or not but when sneaking and using flexible block placement, the player will stop sneaking when pressing your block placement hotkey.

Current mod list:

I am using a mouse with a numpad (Scimitar RGB Elite) and I'm using keys 4, 5, 6 for (rotation, offset, adjacent). These get mapped to KP_4, KP_5, KP_6. Tested with keyboard (K55 RGB Pro) numpad and getting same results. Testing with B, N, M works as expected (able to sneak and place). Seems to me that the inputs for the numpad are different than the rest and is getting interpreted as rapid on/off rather than being held down.

What MC version and platform are you playing on? This doesn't seem to happen to me in my test instance. But try setting the Cancel further processing setting to false in the Advanced Keybind Settings for those flexible activation keys (rotation, offset, adjacent, whichever you use).

Minecraft 1.18.2 (fabric loader 0.13.3) on PC Win11. Tried Cancel further processing -> false, still causes player to stand. I can record a quick vid to demonstrate what's going on. Seems as if it can't decide whether to sneak or flex place and randomly switches between the two.

For me both the numpad (tried KP_4) and the mouse back key (MOUSE_4) and forward key (MOUSE_5) on a Logitech G400 work correctly. None of the keys I've tried have made me drop out of sneak mode when using the hotkey or when also clicking with the mouse to place the block. (I'm on Arch Linux.)

Also do you mean you only drop out of sneak mode when you also click with the mouse to place the block? (Assuming you even still have item use on right click like by default?) It couldn't be an issue with the specific keyboard model you have? Although I guess it's unlikely there would be two-key combinations that conflict even without n-key rollover(?) But also if it only happens when you also right click with the mouse... hmm.

But in any case I don't think it can be anything in the mod code, especially if it doesn't happen when you actually click the flexible placement key, and even if it's set to cancel further processing, but rather it only happens later when you actually place the block. Maybe just to make sure and rule things out, can you try without Cloth Config and also without Fabric API (as malilib, Mod Menu and Tweakeroo alone don't need those). And also maybe try with a fresh Tweakeroo config (rename the actual config or move it out of the config directory) where you just enable and bind the Flexible placement keys. (Wait does the Curse launcher not even allow removed the Fabric API? Or why is it missing the enable toggle and remove buttons in the image above...)

What other mods do you have that deal with keybinds in some way? Have you tried with just malilib + Tweakeroo if it still happens?

I leave sneak mode the moment I press the hot key, not necessarily only when I try to place a block. I did try a Dell KB216 with same results, numpad keys don't work as expected.

Tried first with all mods disabled except for Fabric API, Tweakeroo, and malilib, same results. Curse launcher doesn't allow disabling Fabric normally, however I manually changed it to disable by going into mods folder and adding ".disabled" after ".jar" (that's how Curse does it for other mods). Still getting same results. This was also done with only the flex placement enabled and only changing the placement keys.

Video of bug:
https://youtu.be/s1S9bz1v5M8