Hi Nikolay,
This does seem to solve the issue - but the TB in that TBB style has been set to By Instance since I defined the style, several files ago. I haven't been switching the TB between By Style and By Instance in the style (or in the instance on the sheet for that matter).
When using the TBB tool to insert an instance of the TBB style, the TB would initially be present and operating normally. Then, once I edit the TB within the TBB style a handful of times, the TB seems to disappear apropos of nothing. My edits are all occurring in the graphics and type of the TB itself, not with any By Instance or By Style buttons.
I realize that this is not the normal workflow - usually the TBB instance would be placed in a drawing, and one would edit the instance, instead of the style.
I've been developing this TBB style and learning how this all works, so I've been doing a lot of fiddling about as I learn. 🙂