why did you change the way masks work with lumetri colour? a feature i use everyday.

Here is a HelpX page on this topic. As noted already, Frame mode should work to accomplish your workflow. I hope this helps.
https://helpx.adobe.com/premiere/desktop/add-video-effects/create-masks-and-composites/adjust-masks-at-the-clip-or-frame-level.html

For manual animated mask you need to be in frame mode not clip mode.

There are now two types of masking operation keyframe, clip and frame I think it is. If you have it in clip 'mode' then it doesn't allow manual tracking. I'm not at the computer and I've not done that yet so I can't point out exactly where the control is.

But look for information on masking tracking in 26. There's a couple threads here, and also there have been posts linking to explanations of this new behavior in the 'manual' online help.

as in, putting keyframes on the mask path - isn't working. whenever i make changes to the mask path - it doesn't remember the previous mask path when i've made a keyframe.

I'm trying to mask a lumetri colour on a small, moving part of the frame. The stedicam operator tripped up on the textile - so i'm trying to darken a small part of the frame - manually changing the mask shape over about 20 frames. I'm using the new mask feature - I've manually made a mask but it's staying still - even when i add key frames. it feels like it's a bug? 

Look to Matt Christenson's explanation and demo in this thread:

https://community.adobe.com/t5/premiere-pro-discussions/this-new-mask-tool-is-awesome-but-how-do-i-mask-with-the-pen-tool-instead-of-creating-shapes/td-p/15677063

We would need more details to help ... although one thing catching a number of people is that masking is now in the Tools palette of the Timeline ... and that may be the problem you're having. 

Down in the Timeline Tools panel along with Ripple and the Hand tool and such there's a masking icon, and that leads to several masking options.