It sounds like you are using the Modifier selection to perform this setup, I would suggest that you use Keystrokes to add the Shift+Options like the image below shows.
Let me know how this works for you.
That will work. The image below shows you how to set it up.
Open the Setting panel, and  select the 3-Buttton pen.
 If you have added an application to the Application that you are wanting to use the function on, like Adobe Illustrator, click the icon and they will see the yellow bar appear under it.
 Click the pen button you are wanting to use.
 Give it a name.
 Clear and then click the cursor in the Keystroke keys field, and then press the Shift key.
 Once done, click OK.
And you are set.
Maybe there's a miscommunication. You're asking me to add the shift modifier to the pen button. I simply want to figure out why the shift key on my keyboard is unreliable when used WITH the pen tablet. This solution would have me use the pen, then hold down the pen button, then use another modifier key on my keyboard(i.e shift+option+F5).
This isn't really a viable solution. It's a workaround. Again, is this an issue with the driver?
Sounds like the misunderstanding is on my side...
I see what you are talking about. When I add Shift+Option+5, to the keystroke field I get Shift+Option+%. I'm going to talk with the Driver team about this and see what we can come up with.
Why your keyboard is not responding to this reliable, I'm not sure.
If you are able to remap that key combination to another keystroke that would be the quickest solution.
I have the same issue on MacOS in both Houdini and Silo which makes the tablet pretty much unusable since both Apps use shift or control frequently as modifiers for selecting nodes / geometry.
I put in a ticket for it (#2453), but looks like it's fallen through the cracks - it's bee about 8 months since an update and any enquiries go unanswered. So, at the moment, the tablet is just a rather expensive paperweight. Would be nice to know if this is an issue that the is being looked into at all. Either way, it's not an isolated issue and affects Illustrator, Houdini and Silo (think Blender was OK...).
Sorry for the late reply on your ticket. That was my fault!
I just replied to you via the ticket. We have had some macOS releases since then that will get you up and running.
I have the same problem. Modifier buttons are not recognized or remain even though the button is no longer pressed. MacOS Ventura 13
Thanks for your post, and this issue should be fixed in the next macOS driver release. And that should be released in the next two to three weeks.
Just in case someone else comes across this thread; looks like everything is working well in Houdini with the latest release of the drivers (12.2.1-40).
Thanks for reporting that. If anyone comes across any issues in Houdini, please contact CustomerCare@xencelabs.com.
Hello, I am currently having an issue when using the modifier on the keyboard with the pen. It appears that the shift key is inconsistent when used with the pen. While using Adobe Illustrator, this can be very frustrating when trying to use multiple modifiers(like shift+option).
It has been suggested that I assign the shift to one of the buttons on the tablet, but it's not a proper solution since I will use more than one modifier key at once-and altering my workflow for one key is not really a solution.