Search failure

I have a document with the following text that seperates chapters:

= = = = = =

I copied that from the document and then pasted into the search box and it found 55 occurances within the document.

HOWEVER... after a few hours reading through the document and using the search to go back and forth between various chapters the search reports "No matches found" 

My workaround is to exit Acrobat Reader and reload the document, then search can find the text again.

My system is comprised of an intel core i7 with 16GB RAM running 21H1 version of Windows Pro and Acrobat Reader version 21.005.20060  (it reports no updates available)

The file has the following specifications as reported by Acrobat Reader:

PDF Producer:  Acrobat Distiller 4.04 for Windows

PDF Version:    1.3 (Acrobat 4.x)

File Size:           3.27MB (3,429,890 Bytes)

Page Size:          8.50 x 11.00 in

Pages:                 345

Taged PDF:         No

Fast Web View:   Yes

Should I report this as a bug?