We use Acrobat Pro 2017 Classic on MacOS High Sierra. Before the school holidays, we bought a license for Acrobat Pro 2020 Classic that is installed on the same machine.

We mainly use Acrobat to OCR screenshots and combine them into one PDF. Such a PDF contains between 40 and 200 OCRed pages. Sometimes we use TIFF, sometimes we use PNG as source images that need to be "PDFed".

The workflow is this:
1- Combine source images into one PDF in Acrobat Pro
2- Crop borders of each page in Acrobat
3- Export each page to a directory as a separate pdf file (1)
4- Close (and discard) combined PDF
5- Batch OCR each PDF file in source folder and save the OCRed pdf page in another directory
6- Combine OCRed PDF into one PDF
7- Safe PDF

(1) We do this because we discovered that the OCRed results are better when we ask Acrobat to OCR page by page instead of per multi-page document. Next to this Acrobat has a limit of OCRing 500 pages.

Now that we have Acrobat Pro 2020, we noticed that Acrobat Pro 2017 sometimes mixes up page order. Source images normally have a number like IMG0626.PNG, IMG0627.PNG, IMG_0628.PNG, etc.

The error in Acrobat 2017

This mixup error happens in workflow step 1 when source pages are combined into one PDF in Acrobat Pro. It doesn't matter if we import PNG of TIFF files. The error remains.
In the test set of 46 pages, pages 24 and 25 are swapped. In larger file sets there can be several spontaneous mixups of pages.

Q: Are the sources pages correctly sorted (so page 24 and page 25 are not swapped)?
A: yes. Page 24 and 25 are NOT swapped in the source directory. They are sorted as expected.

Q: Will the same error happen when I replace the file names of the PNG of TIFF files by a sequence number nnn so we get 001.PNG, 002.PNG, 003.PNG?
A: yes

Q: Will the same error occur when I turn toggle bookmark creation when combining?
A: yes

Q: When you combine the OCRed pages back into one PDF, does the mix up happens there also?
A: no

Acrobat 2020 is dead slow combining a pdf (compared to Acrobat 2017). Is it caused by a bug?

Importing a 85MB PNG file takes 3 to 4 seconds per page in Acrobat 2017. In Acrobat 2020 it takes 10 to 11 seconds.
Importing that same source file of about 100MB in (uncompressed) TIFF takes 3 to 4 seconds In Acrobat 2017. In Acrobat 2020 it takes 14 to 16 seconds.
Combing PDF files goes fast in both Acrobat Pro releases.
In both cases Acrobat Pro is installed on the same Mac Pro 2010 running High Sierra on a 1TB SSD disk with plenty (+/- 30%) of free storage. I did reset my Adobe preferences as suggested in the Adobe Acrobat forum.

I really hope that a bug is causing Acrobat Pro 2020 Classic for taking its time to combine source images to a PDF.

OCRing

What seem to go faster is OCRing in Acrobat 2020 compared to Acrobat 2017 when OCRing to editable text and images. OCRing the same set in Acrobat 2017 takes about 32 to 52 seconds. In Acrobat 2020 it takes about 11 to 20 seconds. To be OCRed text language is Dutch.

Rare crash of Acrobat Pro when OCRing

Another error that result in a crash of Acrobat Pro is that it fails to OCR a page. It simply crashes. My source pages have a border that I crop away before OCRing. In most cases when I keep the border at a page Acrobat Pro crashes, Acrobat Pro is -- in most cases -- able to OCR the page.

This is a rare crash. I did not yet come to a page so I can test it in both Acrobat Pro 2017 and 2020 to see if this is a bug related to one or both versions of Acrobat Pro.

I am happy to provide the support team the test set that I used (compressed to a 173MB ZIP file). Unfortunately it contains copyrighted material so I can't attach it just to this ticket.