Hello,
 
I am using a Windows 10-upgraded studio PC and Sonar Producer 8.5.  Since the OS upgrade, the Cakewalk Modfilter plugin now acts strangely: I normally set to "manual" w/Tempo Sync off and automate the cutoff filter min-to-max for a desired effect.  However when I use it in projects now, it no longer behaves: is stuck with 'tempo sync' on and automating cutoff from low-to-high or reverse only results in jerky on/off fashion of the envelope (maximum and minimum only)
 
I set tempo sync to "off" and the 'wah-wah' tempo throb continues - seems unresponsive. This plugin is the only one that seems to be misbehaving.  The rest of the DAW software seems to be working fine for me.... Is this a software issue and my Sonar finally needs an upgrade? 

You need to upgrade. LOL I have no idea how to approach this because I don't have 8.5 on my system. 

I'm not using 8.5, but I'm using Windows 10 with Platinum.
 
Moving the Cutoff knob manually works okay going from Min to Max, but going from Max to Min exhibits the on/off behavior you describe. Fortunately, there are two workarounds.
 

• Automation envelopes work as expected. IOW if you draw a downward slope, the parameter will change as desired.
• The Assignable Controls option works, so you can move the slider in Console view or the Inspector Track view and control the parameter normally. Even better, you can remote control it with a mod wheel (or whatever) for even more precise control by using the Assignable Controls remote control option.

 
If you're not familiar with Assignable Controls, see Week 17 in the Friday's Tip of the Week thread.