Undo does not work properly on deleted text in InDesign 19.4

Just to make sure we're talking oranges and oranges, Undo doesn't really care about content or context — it un-does each action in reverse order. If you deleted a block of text with Backspace or Delete, it will undo each of those key actions, one at a time. If you selected a block of text and hit delete, it will undo that action, restoring the whole block at once.

Put another way, it doesn't know you want to undo the last 43 Deletes as a group. (Although some apps over the years seem to have had such "intelligent" undo.)

What OS and version number?

have you tried these fixes?

Try resetting your preferences:

Windows: Start InDesign, and then press Shift+Ctrl+Alt. Click Yes when asked if you want to delete preference files.
macOS: While pressing Shift+Option+Command+Control, start InDesign. Click Yes when asked if you want to delete preference files.
A dialog will appear asking to delete the InDesign Preferences, select 'Yes' on this dialog.
Further info and instructions here if needed:
https://helpx.adobe.com/in/indesign/kb/indesign-preferences-support-file-locations.html