tiff files no longer editable as smart objects

"It duly opens the file, but since the latest edition(s), will not make any alteration to that file - stating that it "could not use the healing brush because the smart object is not directly editable"."

No, this is not new, this is how smart objects have always worked. A smart object cannot be directly edited on pixel level. You cannot use paintbrush, clone stamp, healing tool etc. It doesn't matter if the contents of the smart object is a raw file or an RGB file.

Put another way, you can't use local tools. You can only use global tools, and mask them.

To do local pixel edits, you have to open the smart object and work on the original embedded file.

My original point is still valid. There is no difference between creating a smart object from an RGB file in Photoshop, and sending that RGB file to Photoshop as a smart object from Lightroom.

The only possible difference is that you can send the file from Lightroom with Lightroom adjustments baked in. But you can't then return to those adjustments later, the way you can with a raw file. They will be baked into the pixel data as a single flat layer. You can, however, use the ACR filter in Photoshop to do the very same adjustments, and these will be re-editable in a smart object made in Photoshop.

Ah, hold on. There is a workaround. Set "TIFF handling" in ACR Preferences to "always open". Apply some Lightroom adjustments to a TIFF, and then open it as smart object in Photoshop. This does in fact open the TIFF in ACR.

Still, my previous point applies. There is no real reason for doing it this way, no benefit or advantage. You can just as well do it all in Photoshop. That would probably also make it a lot clearer what actually happens.

Why would you send a TIFF as smart object from Lightroom? That's a rendered RGB file, not a raw file. This isn't what this workflow is intended for. Lightroom adjustments to an RGB file are metadata that Photoshop cannot read. To get the Lightroom adjustments into Photoshop, it has to be in the form of a processed copy with the adjustments baked into the pixel data.

This is very different for raw files, because you get an embedded raw file that can be reopened in ACR later.

If you want this TIFF as a smart object, create the smart object in Photoshop. There is nothing added by doing it from Lightroom.

You can use the ACR/Lightroom controls on an RGB file, but then you'd do it using the ACR filter (which is not the same as the ACR raw processor).