You are right, the + sign seems to have disappeared from the editor space itself.
It actually happens with the default themes, too.
I'm looking into it to understand the details.
In the meantime you can insert blocks using the + button at the top left of the screen, or by using the / key after pressing return to add an empty paragraph block.
Testing this further, I'm not sure how much of an issue it is.
In the following screenshots I have the Kadence theme active, alongside Toolset Blocks.
In screenshot 1, I have a header block inserted and it is the currently highlighted block. The + button to insert a new block is visible (and works).
In screenshot 2 I hit enter to insert a new block (rather than use the + sign), which by default inserts a paragraph block. It is true that the + sign to insert a block is missing while this paragraph block is selected: you can change the block to some other block by pressing the / key or using the + button at the top left (from the screenshot I shared in the previous reply).
Once you've added some block, the + sign re-appears to add another block (unless you hit enter and insert a paragraph block, then have to switch it to another block).
On the basis of the above I don't think there is a clear issue for me to escalate.