Why are characters in text suddenly missing in InDesign & best way to fix

Help, a most bizarre occurrence recently. 

I am doing the design, layout, and typesetting of an 88-page magazine using InDesign (2025) 20.4.1 with a M4 Mac mini and MacOS Sequoia 15.3.2 (24D81).

The InDesign file is located in my hard drive Dropbox folder synced to my Dropbox on the Dropbox website.

After finishing a second round of the entire magazine (after making corrections) and sending proofs to staff, etc, suddenly 3 consecutive letters are missing throughout the entire document when they were not previously. They are missing in the InDesign file and the PDF I made afterward. The original sin was in InDesign.

The missing letters are all  “med” in this consecutive order. For example, words like “medical,” “Medicare,” “Medicaid,” “deemed,” “confirmed,” “intermediary,” etc suddenly are missing and leaving in the text “icaid” “care” “dee” “confir” “interiary” etc.

All the 3 letters that were missing were the letters “med” in this consecutive order. For example, words like “medical,” “Medicare,” “Medicaid,” “deemed,” “confirmed,” “intermediary,” etc suddenly were missing. 

I’ve fix all the errors laboriously via spell checks, reading every article, and search and replace.

I’ve seen the following as a workaround solution: “…the fastest fix is a Ctrl-A and when 'all' is selected change the color or the font or the size and it seems to reset it.”

The above is not a satisfactory solution; it does it address the reason why it happens so that it can be avoided in the future.

The question is: Why were these 3 particular letters missing throughout the document?

Desperate for anyone's help. Thanks so much.