At some point recently (not sure when, but I just notice it now when I opened up a program I had set to auto-open a mirrored window with triggers that runs functions to move and resize it), window resize functions (whether automatically through triggers or manually with hotkeys) no longer work on borderless Mirror Windows. It seems to still work on ones with borders, though.
I'm currently on Windows 10 Pro with Version 10.0 (Beta 13) of DisplayFusion.

I have two trigger rules. One to create the Mirror Window, and then another to move and resize the created window. Resizing sudden stopped working on borderless Mirror Windows even when trying to apply it manually (but the repositioning function still works), so I don't think it has anything to do with the trigger rules specifically. If I change the trigger rule to create a bordered Mirror Window, instead, then the resizing happens as expected.

Well, looks like borderless mirror windows can now be resized by Functions, again. But whatever was done to fix this now made it so all of my Trigger rules to auto-generate the mirror window, as well as the Trigger to move and resize the mirror window when it gets created, no longer work.

It's pretty weird that my Trigger settings that worked prior doesn't work now. I changed the rules a bit multiple times to see if I could get it working (but failed), but I made sure to change them back to what they were originally.

Actually, I just realized that ALL of my Triggers aren't working since upgrading to Beta 14.

One of the beta releases in the past month once again broke the trigger that I have set to move and resize mirrored windows again. I think I might have skipped a version or two of the beta, so I'm not sure which version broke it.

Here you go.

Yes, all functions seems to work when applied manually. I tried adding a delay of half a second to the trigger just now, but that didn't make the trigger work, so I don't think WHEN the function is applied is the issue.

The former. But I just applied the latter now as well, and that changes nothing.

I have pinpointed the problem. I had the Trigger conditions set to only apply to a certain Monitor Profile, which caused it to not trigger despite being on that monitor profile. Turning off the condition for Monitor Profile makes it work as intended. No delay, or any other conditions I tried to add to fix it previously, is needed. So the bug is with the Triggers not detecting monitor profiles correctly, it would seem.

On a side note: why is it that when you create a mirror window for any Window, it doesn't have a black border on the mirror window when the parent window is not maximized, but when you maximize the parent window, the mirror window adds a black border (making the display area of the mirror window smaller as a result)?

It doesn't indicate what the profile is on the upper-left. But even loading the designated Monitor Profile of the trigger, despite being already on it just to make sure, right before triggering the trigger doesn't do anything.

That is not what's going on. There's a black border an all four sides, roughly 10-15 pixels thick. I mirrored a 1920x1080 window on my display tablet onto the mirror window on my bigger main monitor, and made sure down to the pixel that it's mirroring the pixels on a 1-to-1 scale. When the parent window is made to span 1920x1080 while not maximized, the appropriately sized mirror window mirrors it 1:1. But when I maximize the parent window on the 1920x1080 monitor, the mirror window suddenly adds a thick black border on all four sides.

Attached is my Monitor Configuration window.

I created a new profile of my default layout called "Test", and it says "Detected Profile: Test" in the top-left. If I load a different layout, it would go away, as expected. But if I load my old profile that has the exact same layout as my Test profile, it would also display "Detected Profile: Test". So I guess profiles created before a certain version no longer gets detected, despite one still able to load them.

Also, it turns out that the mirror issue with the black border on maximize only happens when I try to mirror Adobe Photoshop. So the way Photoshop (or at least my version of it, which is CC 2019) handles its window doesn't play nice with DF's code. That would also explain why you haven't been able to replicate the issue, since you probably didn't test it with Photoshop.

Just so you can see what I see: here are two screenshots of my relevant monitors, one with the parent window not maximized but made to span the whole monitor, and the other with it maximized. The larger monitor on the left has the mirror window (to the left of the Photoshop Navigator sub-window) and the parent window is on the smaller monitor on the right. Also, I just noticed that when I maximize the parent window, it puts a red outline around it for whatever reason.
(The short, cyan line at the top of the larger monitor is just Window's screen-snipping UI hiding itself as I captured the two monitors, so that's not part of the bug.)