Sometimes this is a formatting conflict or a hidden page break character that causes threaded text to go into overset mode. Is the text in your 20 pages threaded? By clicking your Type tool into the thread of text and then turning on Edit > Edit in Story Editor (Cmd/Ctrl+Y) you can sometimes spot the hidden character. In Story Editor mode, you can also examine which paragraph style is at work. By looking at the Paragraph Style Options, you might discover a conflict in, maybe, the Keep Options, for example.
Hi @marcelo_8816, just following up on this conversation to check if you were able to try the steps suggested by Abhishek and Dave, and if they helped?
If not, please let us know and we’ll be happy to help further.
Thanks for writing to the community, and I'm sorry you're experiencing this issue. Could you please confirm your operating system and version? Also, were you able to try disabling GPU Performance as suggested by the Expert, and did that make any difference?
If you're still seeing the issue, please try running Recompose All Stories using Quick Apply (‘Cmd/Ctrl +Enter’), type Recompose All Stories, and press Enter. Then, force a screen redraw by switching between Normal and Preview mode (W key) or by zooming in and out to see if the missing text reappears. If the issue still persists, please try exporting the document as IDML, reopen it, and save it as a new INDD file, as this can help resolve document-related corruption issues.
Please let us know how it goes after trying these steps, and we'll be happy to investigate further.