Tabs cannot add inner blocks

Steps to reproduce:
* create a tabs block
* in a tab, create a Columns Manager block with 3 columns
* in a Column, create a core paragraph block
* repeat for the rest of the 2 columns

Expected results
* tab with 3 columns, each with a paragraph

Actual results
* cannot create paragraph blocks – no error message; simply cannot create the inner block

Versions
* fresh 5.5.1 install
* theme 2020
* plugins – none, only Advanced Gutenberg 2.4.7

Thanks!