Hi @JohnBSL
I encountered the same problem - for me, the dynamic content is not deleted, it is just hidden and the Flow is working correctly.
The only solution others seem to have provided is disabling experimental features. As I enjoy the expanded expressions editor provided by experimental features, I have found a work-around that means you don't have to disable them.
1) Select the textbox where your dynamic content is missing, click Ctrl+A, to select all, then copy the selection
2) Paste it into a .txt document (like Notepad) - you should see code relating to your dynamic fields
3) Select everything in the Notepad file (Ctrl+A), copy it and paste it back into the Power Automate textbox
4) Your dynamic content should appear correctly.
You'd have to do this every time you close/open the Flow, for every textbox, but if you want to keep your experimental features, it is a quick workaround until Microsoft fixes the issue 🙂