There's something weird with headings that can cause unfolding to fail. Headings can be folded with TAB refuse to unfold - they need Shift-TAB to see the contents. This is scary as it looks like content has been accidentally deleted whereas it's really just hidden.

The TAB-cycling worked as expected in Org 9.1.9 for the following example but fails in 9.4.

The example below illustrates the difficulties:

• Block 1 - you can delete X to leave two empty lines and TAB on "Block 1" (or 1a) cycles as expected - GOOD!
  

But if the last line isn't empty but has spaces:

• Block 2 - deleting X (leaving empty line) and then Z to leave the last line with just a space (one or more) and heading will fold on TAB but won't unfold. Shift-Tab is needed.
  
• Block 3 - the same symptoms as Block 2 but has a subtree to show "Folded/Children/Subtree" when Tab-cycling on level one heading.

Cheers
Giovanni

===================

* Block 1 - OK
Block 1 text

** Block 1a heading
Block 1a content
X
X
* BAD BLOCK 2 - Delete X & Z, then TAB WON'T UNFOLD
Block 2 content
X
 Z
* BAD BLOCK 3 - TAB WON'T UNFOLD
  Block 2 content

** BAD BLOCK 3a
   Block 3a content
X
 Z
* Block 4 - OK
The end